[LSP]: add code-completion

[GitMensch]

ideal would be context-sensitive code completion, but a general list of the words in the tmLanguage file alone would already be useful; it seems this would be done by languages.registerCompletionItemProvider.

[pirpyn]

Never done it, i'll have a look at the doc. I'll see in 2-3 days.

[pirpyn]

This will needs a Language server. We can create one from the vscode documentation.
I have to evaluate how much time it could take.

[GitMensch]

If possible, it would be good to implement that language server purely in javascript, so it can run in the editor = client-side extension-host.