LSP - add `documentHighlight` feature

Document Highlights Request

The document highlight request is sent from the client to the server to resolve a document highlights for a given text document position. For programming languages this usually highlights all references to the symbol scoped to this file. However we kept ‘textDocument/documentHighlight’ and ‘textDocument/references’ separate requests since the first one is allowed to be more fuzzy. Symbol matches usually have a DocumentHighlightKind of Read or Write whereas fuzzy or textual matches use Textas the kind.

https://microsoft.github.io/language-server-protocol/specification#textDocument_documentHighlight

Start by reading ALE's existing Language Server Protocol request handling and the linked documentHighlight specification. Compare the proposed request with the project's existing reference-related behavior, then identify the relevant implementation and test entry points. Done means ALE supports textDocument/documentHighlight and returns the expected read, write, or textual highlights.

Written by the indexing model from the issue text.