Autocomplete for function calls suggests labeled args that were already assigned

[cknitt]

(Followup to #175)

This was already solved for JSX props, but is still an issue for labeled args.

Example (only _to should be shown in the list as from was already assigned a value):

[cristianoc]

About what's involved:
The backwards parser for function calls currently only deals with nested ( and ), so it goes back to substring( and completes from it. So it never parses from.
What's needed is to parse all the arguments between substring( and ), with all the possible forms they can take, including args included in parens, brackets, etc, optional args, type annotations, punning.

[cristianoc]

Here: https://github.com/rescript-lang/rescript-vscode/blob/master/analysis/src/PartialParser.ml#L38