fix(outline): use the correct symbols and filter config format (#3924)

## What is this PR for?

The symbols-outline extra was removed in favor of outline.nvim in #2535
(thanks!), but the configuration for symbols in outline.nvim [is not
backwards-compatible](hedyhli/outline.nvim#12).

This fixes the configuration for the symbols icons and filter to be
usable by outline.nvim.

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  why we should accept this pull request. -->

## Does this PR fix an existing issue?

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  If this PR fixes any issues, please link to the issue here.
  Fixes #<issue_number>
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It doesn't seem like anyone has encountered this issue, but I can
confirm that the config currently used by LazyVim is incorrect. The
symbols table is at `symbols` for symbols-outline.nvim, but it's now at
`symbols.icons` for outline.nvim.

There is no such `symbols_blacklist` key. Instead, `symbols.filter` is
used, which is a kind of "whitelist".

Coincidentally, outline.nvim fully supports the LazyVim `kind_filter`
config table structure. It can either be a list of strings (kinds), or a
list of strings for each filetype key. Setting to nil or false makes it
so all symbols are included, just like in LazyVim.

See [the docs on the
`symbols.filter`](https://github.com/hedyhli/outline.nvim?tab=readme-ov-file#symbols-table)
structure.

## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/plugins/extras/editor/outline.lua

@@ -14,28 +14,21 @@ return {
     opts = function()
       local defaults = require("outline.config").defaults
       local opts = {
-        symbols = {},
-        symbol_blacklist = {},
+        symbols = {
+          icons = {},
+          filter = LazyVim.config.kind_filter,
+        },
         keymaps = {
           up_and_jump = "<up>",
           down_and_jump = "<down>",
         },
       }
-      local filter = LazyVim.config.kind_filter
 
-      if type(filter) == "table" then
-        filter = filter.default
-        if type(filter) == "table" then
-          for kind, symbol in pairs(defaults.symbols) do
-            opts.symbols[kind] = {
-              icon = LazyVim.config.icons.kinds[kind] or symbol.icon,
-              hl = symbol.hl,
-            }
-            if not vim.tbl_contains(filter, kind) then
-              table.insert(opts.symbol_blacklist, kind)
-            end
-          end
-        end
+      for kind, symbol in pairs(defaults.symbols.icons) do
+        opts.symbols.icons[kind] = {
+          icon = LazyVim.config.icons.kinds[kind] or symbol.icon,
+          hl = symbol.hl,
+        }
       end
       return opts
     end,