Overhaul TextMate syntax highlighting

[rvanasa]

Continuation of #23. The current TextMate language specification is almost directly based on Swift, so this PR fixes most of the remaining syntax highlighting and namespace issues.

This PR also adds a minimal .vscodeignore file as encouraged by the vsce package command.

Works as expected on Ubuntu (WSL) using the latest version of VSCode.

[rvanasa]

(@kritzcreek)

[ggreif]

I guess this is fine. I don't know much about TextMate grammars, but if this floats your boat, then you have my blessing! We can certainly improve this iteratively if issues show up.

Thanks for the contribution!

[rvanasa]

Thank you! The grammar file will definitely require a few more iterations, but at least this is now on par with the syntax highlighting in Motoko Playground.