editor: Implement Go to next/prev Document Highlight (#35994)

Closes #21193
Closes #14703 

Having the ability to navigate directly to the next
symbolHighlight/reference lets you follow the data flow of a variable.
If you highlight the function itself (depending on the LSP), you can
also navigate to all returns.

Note that this is a different feature from navigating to the next match,
as that is not language-context aware. For example, if you have a var
named foo it would also navigate to an unrelated variable fooBar.

Here's how this patch works:

- The editor struct has a background_highlights.
- Collect all highlights with the keys [DocumentHighlightRead,
DocumentHighlightWrite]
- Depending on the direction, move the cursor to the next or previous
highlight relative to the current position.

Release Notes:

- Added `editor::GoToNextDocumentHighlight` and
`editor::GoToPreviousDocumentHighlight` to navigate to the next LSP
document highlight. Useful for navigating to the next usage of a certain
symbol.

crates/editor/src/actions.rs

@@ -493,6 +493,10 @@ actions!(
         GoToTypeDefinition,
         /// Goes to type definition in a split pane.
         GoToTypeDefinitionSplit,
+        /// Goes to the next document highlight.
+        GoToNextDocumentHighlight,
+        /// Goes to the previous document highlight.
+        GoToPreviousDocumentHighlight,
         /// Scrolls down by half a page.
         HalfPageDown,
         /// Scrolls up by half a page.