multi_buffer: Make anchor_in_excerpt fallible for bad text anchors (#40496)

`MultiBuffer::anchor_in_excerpt` currently just wraps the given text
anchor in a multibuffer anchor. This allows one to get a multibuffer
anchor that points outside its excerpt which is basically never what one
wants. This PR now does a bounds check and returns `None` if the given
text anchor is not within the bounds of the excerpt.

Release Notes:

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

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/editor/src/items.rs

@@ -364,10 +364,9 @@ impl FollowableItem for Editor {
     ) {
         let buffer = self.buffer.read(cx);
         let buffer = buffer.read(cx);
-        let Some((excerpt_id, _, _)) = buffer.as_singleton() else {
+        let Some(position) = buffer.as_singleton_anchor(location) else {
             return;
         };
-        let position = buffer.anchor_in_excerpt(*excerpt_id, location).unwrap();
         let selection = Selection {
             id: 0,
             reversed: false,