doc: update changelog

Author: tomlau10

changelog.md

@@ -31,6 +31,7 @@
   cfgs[2] = {}        -- only warns missing `b`
   ```
   This enables the previous missing field check behavior before [#2970](https://github.com/LuaLS/lua-language-server/issues/2970)
+* `NEW` Added variable substitution support for vscode's `${workspaceFolder:x}` when resolving path placeholders [#2987](https://github.com/LuaLS/lua-language-server/issues/2987)
 
 ## 3.13.5
 `2024-12-20`