Extract logic around custom text highlights out of InlayChunks iterator (#22104)

This is a pure refactor, extracted from
https://github.com/zed-industries/zed/tree/new-diff-map

Release Notes:

- N/A

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Agus <agus@zed.dev>

crates/editor/src/display_map.rs

@@ -19,6 +19,7 @@
 
 mod block_map;
 mod crease_map;
+mod custom_highlights;
 mod fold_map;
 mod inlay_map;
 pub(crate) mod invisibles;