Implement Signature Help for qmlls

The language server protocol provides support for completions/help when writing a function call.

This is provided through https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_signatureHelp

Currently we offer completions or functions that have the signature, but the better solution would be to provide signature help.

See https://git.qt.io/famohame/lsp-description for a high level overview of qmlls.

This jumps a bit in the deep end, but all things for it are available.

There are two options (when either "(" or "," trigger it):

1) wait to have the current document fully parsed (smaller but less responsive, and fails I for example parens are not yet closed)

2) try to recover the argument and function with an ad-hoc parsing to the left of the cursor (going back), and look that function up

Option 2 is more robust and faster, so I would go with that...