Consider highlighting grammar for function signatures

## Context

As part of https://github.com/hashicorp/terraform-ls/issues/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.)