Use a custom icon for GDL files

[runxel]

Have a custom icon for GDL files, so it doesn't has the generic file icon any longer.

See XML on the left, GDL with the generic icon on the right (current situation):

This issue was to be fixed in the VSCode repo, which is now done. microsoft/vscode#118846
It's still a proposed API, so we can wait a bit for it to become stable.