editor: Some more semantic highlighting perf work (#49491)

- Reduce some re-allocations in `text_anchors_to_visible_anchors` and
`refresh_semantic_tokens`
- Process `raw_to_buffer_semantic_tokens` work on the background thread

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/multi_buffer/src/multi_buffer.rs

@@ -4109,9 +4109,10 @@ impl MultiBufferSnapshot {
         &self,
         anchors: impl IntoIterator<Item = text::Anchor>,
     ) -> Vec<Option<Anchor>> {
+        let anchors = anchors.into_iter();
+        let mut result = Vec::with_capacity(anchors.size_hint().0);
+        let mut anchors = anchors.peekable();
         let mut cursor = self.excerpts.cursor::<Option<&Locator>>(());
-        let mut anchors = anchors.into_iter().peekable();
-        let mut result = Vec::new();
         'anchors: while let Some(anchor) = anchors.peek() {
             let Some(buffer_id) = anchor.buffer_id else {
                 anchors.next();