Consolidate category and labels fields in glossary (#6176)

I originally felt like the separation was useful, but it
quickly became murky as I began to add more entries that fit well into
multiple categories. We don't use these yet, but let's keep it simple
and have one field. If we want to surface some predetermined
"categories" to filter by, we can separately specify important labels.

This PR also adds an extra new line between each entry for some visual
separation while editing.

src/_data/glossary.yml

@@ -46,12 +46,13 @@
       link: "/language/variables#final-and-const"
     - text: "Don't use const redundantly"
       link: "/effective-dart/usage#dont-use-const-redundantly"
-  category: "language"
   labels:
+    - "language"
     - "const"
     - "understanding diagnostics"
   alternate:
     - "Implicit const"
+
 - term: "Definite assignment"
   short_description: |-
     The determination of whether a variable has definitely been
@@ -149,10 +150,11 @@
       link: "https://github.com/dart-lang/language/blob/main/resources/type-system/flow-analysis.md"
     - text: "Understanding definite assignment analysis"
       link: "/null-safety/understanding-null-safety#definite-assignment-analysis"
-  category: "language"
   labels:
+    - "language"
     - "flow analysis"
     - "understanding diagnostics"
+
 - term: "Function"
   short_description: |-
     An umbrella term to refer to top-level functions, local functions,
@@ -162,11 +164,12 @@
       link: "/language/functions"
     - text: "Instance methods"
       link: "/language/methods"
-  category: "language"
   labels:
+    - "language"
     - "umbrella term"
   alternate:
     - "Procedure"
+
 - term: "Irrefutable pattern"
   short_description: |-
     A pattern that always matches.
@@ -183,11 +186,12 @@
       link: "/language/patterns#places-patterns-can-appear"
     - text: "Dive into Dart patterns and records"
       link: "https://codelabs.developers.google.com/codelabs/dart-patterns-records"
-  category: "language"
   labels:
+    - "language"
     - "patterns"
   alternate:
     - "irrefutable context"
+
 - term: "Mixin application"
   short_description: |-
     A class created when a mixin is applied to a class.
@@ -227,11 +231,12 @@
   related_links:
     - text: "Mixins in Dart"
       link: "/language/mixins"
-  category: "language"
   labels:
+    - "language"
     - "understanding diagnostics"
   alternate:
     - "with mixin"
+
 - term: "Override inference"
   short_description: |-
     How missing types in a method declaration are inferred.
@@ -298,10 +303,11 @@
   related_links:
     - text: "Inheritance in Dart"
       link: "/language/extend"
-  category: "language"
   labels:
+    - "language"
     - "type inference"
     - "understanding diagnostics"
+
 - term: "Part file"
   short_description: |-
     A Dart source file that contains a `part of` directive.
@@ -313,11 +319,12 @@
       link: "/guides/libraries/create-packages#organizing-a-package"
     - text: "Use library URIs in `part of` directives"
       link: "/effective-dart/usage#do-use-strings-in-part-of-directives"
-  category: "language"
   labels:
+    - "language"
     - "libraries"
   alternate:
     - "part"
+
 - term: "Potentially non-nullable"
   short_description: |-
     A type that is either non-nullable explicitly or
@@ -340,13 +347,14 @@
   related_links:
     - text: "Nullability and generics"
       link: "/null-safety/understanding-null-safety#nullability-and-generics"
-  category: "language"
   labels:
+    - "language"
     - "type system"
     - "flow analysis"
     - "understanding diagnostics"
   alternate:
     - "potentially nullable"
+
 - term: "Public library"
   short_description: |-
     A library that is located in a package's `lib` directory but
@@ -359,9 +367,10 @@
       link: "/guides/libraries/create-packages#organizing-a-package"
     - text: "Public package directories"
       link: "/tools/pub/package-layout#public-directories"
-  category: "packages"
   labels:
+    - "language"
     - "libraries"
+
 - term: "Refutable pattern"
   short_description: |-
     A pattern that can be tested against a value.
@@ -377,11 +386,12 @@
       link: "/language/patterns#places-patterns-can-appear"
     - text: "Dive into Dart patterns and records"
       link: "https://codelabs.developers.google.com/codelabs/dart-patterns-records"
-  category: "language"
   labels:
+    - "language"
     - "patterns"
   alternate:
     - "matching contexts"
+
 - term: "Subclass"
   short_description: |-
     A class that inherits the implementation of another class.
@@ -416,11 +426,12 @@
       link: "/language/extend"
     - text: "Subtype relationship"
       link: "#subtype"
-  category: "language"
   labels:
+    - "language"
     - "type system"
   alternate:
     - "child class"
+
 - term: "Subtype"
   short_description: |-
     A type that can be used wherever a value of its supertype is expected.
@@ -462,12 +473,13 @@
       link: "#subclass"
     - text: "Substituting types"
       link: "/language/type-system#substituting-types"
-  category: "language"
   labels:
+    - "language"
     - "type system"
   alternate:
     - "subtyping"
     - "subtype polymorphism"
+
 - term: "Variance and variance positions"
   id: "variance"
   short_description: |-
@@ -555,8 +567,8 @@
       link: "https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)"
     - text: "The covariant keyword"
       link: "/guides/language/sound-problems#the-covariant-keyword"
-  category: "language"
   labels:
+    - "language"
     - "type system"
     - "generics"
   alternate: