Signature help

[bentsherman]

Adds a basic implementation of signature help for functions, processes, and workflows.

When you type a method name and then (, vscode will show the method signature and docs (similar to hover hint). As you type each argument followed by a comma, the current parameter is highlighted, and can also show docs for each parameter. We could pull param docs from inline comments or @param directives in the method's groovydoc comment.

My main concern with the current impl is that process inputs don't quite fit the expectations of signature help, which expects a conventional signature of func(param1, param2, param3, ...), whereas process inputs are a bit more complex.

I tried to do a multi-line signature for processes to space out the inputs, but vscode still squashes it onto a single line.

Another thing I could try is to do something like PROC( <1>, <2>, <3>, <4> ) for the signature and then show the full spec below that for the current input, which should be possible with the label / documentation of ParameterInfo. But for some reason, the param documentation is only showing for the first param and then goes away. If I can fix this issue, that might be good enough.

Long term, I think the problem will go away with typed process inputs, which should look more like function / workflow inputs:

def func(a: int, b: String, c: boolean) {
  // ...
}

process PROC {
  input:
  meta: String
  reads: Path

  // ...
}

workflow FLOW {
  take:
  samplesheet: Path
  outdir: Path

  // ...
}