This PR adds support for the "document highlights" language server
feature.
This feature allows a client to highlight all instances of a selected
name within a document. Without this feature, editors perform
highlighting based on a simple text match. This adds semantic knowledge.
The implementation of this feature largely overlaps that of the
recently-added "references" feature. This PR refactors the existing
"references.rs" module, separating out the functionality and tests that
are specific to the other language feature into a "goto_references.rs"
module. The "references.rs" module now contains the functionality that
is common to "goto references", "document highlights" and "rename"
(which is not yet implemented).
As part of this PR, I also created a new `ReferenceTarget` type which is
similar to the existing `NavigationTarget` type but better suited for
references. This idea was suggested by @MichaReiser in [this code review
feedback](https://github.com/astral-sh/ruff/pull/19475#discussion_r2224061006)
from a previous PR. Notably, this new type contains a field that
specifies the "kind" of the reference (read, write or other). This
"kind" is needed for the document highlights feature.
Before: all textual instances of `foo` are highlighted
<img width="156" height="126" alt="Screenshot 2025-07-23 at 12 51 09 PM"
src="https://github.com/user-attachments/assets/37ccdb2f-d48a-473d-89d5-8e89cb6c394e"
/>
After: only semantic matches are highlighted
<img width="164" height="157" alt="Screenshot 2025-07-23 at 12 52 05 PM"
src="https://github.com/user-attachments/assets/2efadadd-4691-4815-af04-b031e74c81b7"
/>
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
