Fix bugs on signature help about labelled and optional parameters

[Lucccyo]

When a labelled argument is initiated, signature-help always displays the first labelled argument in the signature, even if it has already been written.
A hierarchy is processed: the list of arguments is first the labelled ones, then the optional ones.
As it always takes the first one, no matter what has already been written, optional arguments are never highlighted, even if they are initiated with a ~.

In the following example, the cursor is after the second ~ on line 2.
Signature-help should set y as the active parameter, since x is already written, but it still highlights x.

let f a ~x ~y t = (a, y, x, t)
let _ = f ~x:43 ~ 1

This PR fixes this wrong behaviour.

[voodoos]

Look slike this also has an effect on another signature help test:

Is that an improvement or a regression ?

[Lucccyo]

It can make the labelled parameter active.
- $ $MERLIN single signature-help -position 2:14 <<EOF
+ $ $MERLIN single signature-help -position 2:15 <<EOF
  > let map = ListLabels.map
  > let _ = map ~f:Int.abs
  > EOF

To make this test correct, the cursor must be placed after the :. The label ~f on its own is a valid parameter and will set the active_parameter to the next argument. However, if the cursor is set after the :, ~f: is no longer a complete parameter. In this case, the active_parameter remains on f, waiting for the value provided after the :.

[voodoos]

Thanks, looks reasonable. one last comment... and we need a changelog entry :-)

[voodoos]

Thanks @Lucccyo !