Feature request: Autocomplete labelled arguments

[zth]

On the same theme as https://github.com/rescript-lang/rescript-vscode/issues/64, I think autocompleting labelled arguments would be very valuable.

So, let someVar = someFunc(~ would autocomplete a list of all (unused) labelled arguments for someFunc.

I've included a sample of what I mean at the bottom of this file:

https://github.com/zth/rescript-editor-tooling-playground/blob/afbdd2a68baedb49929eaa2b9e89bf7114d6a743/src/FunctionHoverIssues.res