Language Service: Implement Semantic Token Provider

[marcoroth]

Opening as WIP.

This pull request implements textDocument/semanticTokens, so an editor colors HTML+ERB from the parsed template rather than from a TextMate/Tree Sitter grammar guessing at it with regexes.

A grammar has to decide what <div class="a <%= b %>"> is by looking at characters. Herb already knows, so the tokens it hands back stay right where nesting gets awkward, and they carry things no grammar could work out at all, like whether a helper came from Action View or from the application.

What it claims, and what it leaves alone

Token	Type
Tag names	type
Attribute names	property
Attribute values	string	quoted and unquoted
< > </ /> <% %>	macro	output modifier on <%=
<!-- --> and <%# %>	comment
<!DOCTYPE>	keyword
Ruby keywords	keyword
Action View helpers	function	defaultLibrary modifier
Names in a locals: declaration	parameter

The Ruby inside a tag is deliberately almost untouched. A Ruby language server describes it far better.