Follow-up semantic token fixes (#48485)

Follow-up of https://github.com/zed-industries/zed/pull/46356

* better selection in highlights panel to show the color of the active
item
* no `DiffbaselessAnchor` entity to cause flickering and highlighting
issues
* react on theme and theme settings change and refresh semantic tokens
* do not eagerly flicker when refreshing the tokens
* do not show semantic tokens in the editor menu for non-applicable
buffers

Release Notes:

- N/A

crates/multi_buffer/src/multi_buffer.rs

@@ -6,9 +6,7 @@ mod transaction;
 
 use self::transaction::History;
 
-pub use anchor::{
-    Anchor, AnchorHasDiffbaseError, AnchorRangeExt, DiffbaselessAnchor, DiffbaselessAnchorRangeExt,
-};
+pub use anchor::{Anchor, AnchorRangeExt};
 
 use anyhow::{Result, anyhow};
 use buffer_diff::{
@@ -7312,23 +7310,6 @@ impl Excerpt {
                 .is_ge()
     }
 
-    fn contains_diffbaseless(&self, anchor: &DiffbaselessAnchor) -> bool {
-        (anchor.text_anchor.buffer_id == None
-            || anchor.text_anchor.buffer_id == Some(self.buffer_id))
-            && self
-                .range
-                .context
-                .start
-                .cmp(&anchor.text_anchor, &self.buffer)
-                .is_le()
-            && self
-                .range
-                .context
-                .end
-                .cmp(&anchor.text_anchor, &self.buffer)
-                .is_ge()
-    }
-
     /// The [`Excerpt`]'s start offset in its [`Buffer`]
     fn buffer_start_offset(&self) -> BufferOffset {
         BufferOffset(self.range.context.start.to_offset(&self.buffer))