Clean up str append/prepend a little (#797)

- Move to directory for other `str` contributions
- Add simple `help` docs
- Simplify type check since only 2 cases are possible

@savente93 Are these okay?

stdlib-candidate/std-rfc/mod.nu

@@ -1,6 +1,6 @@
 # modules
 export module record/
-export module str.nu
+export module str/
 # commands
 export use fs.nu *
 export use set-env.nu *

stdlib-candidate/std-rfc/set-env.nu

@@ -13,25 +13,25 @@
 #   Add a config hook
 #   > set-env -a config.hooks.pre_prompt 'ellie | print'
 export def --env main [
-  field: cell-path # The environment variable name or nested option cell path
-  value: any # The value to set or append
-  --append (-a) # Append to the previous value or wrap in a new list
+    field: cell-path # The environment variable name or nested option cell path
+    value: any # The value to set or append
+    --append (-a) # Append to the previous value or wrap in a new list
 ]: nothing -> nothing {
-  def 'get or' [default field] {
-    get --ignore-errors $field | default $default
-  }
-  let value = if $append {
-    $env | get or [] $field | append $value
-  } else {
-    $value
-  }
-  let field = $field | to text | split row .
-  let value = match $field {
-    [_] => $value
-    [$root, ..$field] => {
-      let field = $field | into cell-path
-      $env | get or {} $root | upsert $field $value
+    def 'get or' [default field] {
+        get --ignore-errors $field | default $default
     }
-  }
-  load-env { ($field | first): $value }
+    let value = if $append {
+        $env | get or [] $field | append $value
+    } else {
+        $value
+    }
+    let field = $field | to text | split row .
+    let value = match $field {
+        [_] => $value
+        [$root, ..$field] => {
+            let field = $field | into cell-path
+            $env | get or {} $root | upsert $field $value
+        }
+    }
+    load-env { ($field | first): $value }
 }

stdlib-candidate/std-rfc/str/mod.nu

@@ -0,0 +1 @@
+export use xpend.nu *

stdlib-candidate/std-rfc/str/xpend.nu

@@ -0,0 +1,39 @@
+# Append a suffix to an input string or list of strings.
+#
+# Examples:
+#   Output 'hello world'
+#   > 'hello' | str append ' world'
+#
+#   Output file names suffixed with '_world'
+#   > ls | get name | str append _world
+export def append [
+    suffix: string
+]: [string -> string, list<string> -> list<string>] {
+    let input = $in
+    let append = { $in + $suffix }
+    if ($input | describe) == string {
+        $input | do $append
+    } else {
+        $input | each $append
+    }
+}
+
+# Prepend a prefix to an input string or list of strings.
+#
+# Examples:
+#   Output 'hello world'
+#   > 'world' | str prepend 'hello '
+#
+#   Output file names prefixed with 'hello_'
+#   > ls | get name | str prepend hello_
+export def prepend [
+    prefix: string
+]: [string -> string, list<string> -> list<string>] {
+    let input = $in
+    let prepend = { $prefix + $in }
+    if ($input | describe) == string {
+        $input | do $prepend
+    } else {
+        $input | each $prepend
+    }
+}

stdlib-candidate/tests/mod.nu

@@ -1,3 +1,3 @@
 export module fs.nu
 export module record.nu
-export module str.nu
+export module str_xpend.nu