Consider highlighting grammar for function signatures

[radeksimko]

Context

As part of hashicorp/terraform-ls#37 we discovered that there's need for rendering function signatures in a markdown hover popup. Because signatures aren't technically valid HCL, the existing grammar(s) do not highlight it particularly well. Specifically, the grammar cannot distinguish between type declaration and parameter name.

Proposal

• Explore the option of a separate small grammar to highlight just function signatures
• Discuss and agree on a stable/predictable way of rendering the signatures - e.g. how to present types (set(string) vs set of string; any single type vs dynamic etc.)