Document ENV variables refs in `workspace.library` config

[hinell]

Add more info on environment variable references for workspace.library config, i.e.:

• ${env:VAR}
• ${3rd}
• ${addons}
• ${workspaceFolder}

They are currently completely undocumented. Few know how to configure .luarc.json properly.

Related:

• luarc.json: Environment variables inside of `workspace.library` paths? lua-language-server#2139

• script/files.lua / function m.resolvePathPlaceholders(...) ... end