Add a `DocumentSymbolProvider` for .gjs and .gts files

[Windvis]

VS Code uses this information for multiple features (and I assume other editors will have equivalents):

• go to symbol
• entries in the "outline view"
• breadcrumbs
• rename symbol (F2) Requires a RenameProvider
• ... (?)

ELS already supports this for .js and .hbs files.

Original issue: lifeart#420

[lifeart]

We could introduce Rename provider https://code.visualstudio.com/api/references/vscode-api#RenameProvider