Allow syntax colouring in parameters for SignatureHelp

[DanTup]

There are a couple of issues that looked like they might cover this:

• Provide API to return formatted content from suggest and parameter hints #11877
• Render IntelliSense suggest item's signature with syntax highlighting #26241

However as far as I can tell, it's not possible to get highlighting in the parameter names in the signature. I've tried various combinations of string/MarkdownString in the documentation properties with no luck. The string being rendered here is SignatureInformation.label which is only a string:

Here the main signature line in the tooltip is just all white. It would be much more readable if the types were coloured like they are in the editor.

[KamasamaK]

I think whatever solution #34188 ends up with would likely work for this UI as well, which I had mentioned in a comment on that issue.

[octref]

Related: #60651