Implement SignatureHelp provider

[trixnz]

Continuation of https://github.com/trixnz/vscode-lua/issues/4.

While we provide argument hints when autocompleting functions, we do not provide them after typing the triggering ( character.

This also ties into https://github.com/trixnz/vscode-lua/issues/5, so some thought should go into how to properly generate signature information from a function declaration while also enriching the suggestions with information from docstrings, if present.