text: Assert text anchor offset validity on construction (#38441)

Attempt to aid debugging some utf8 indexing issues

Release Notes:

- N/A

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/editor/src/highlight_matching_bracket.rs

@@ -29,7 +29,9 @@ pub fn refresh_matching_bracket_highlights(
     if (editor.cursor_shape == CursorShape::Block || editor.cursor_shape == CursorShape::Hollow)
         && head < snapshot.buffer_snapshot.len()
     {
-        tail += 1;
+        if let Some(tail_ch) = snapshot.buffer_snapshot.chars_at(tail).next() {
+            tail += tail_ch.len_utf8();
+        }
     }
 
     if let Some((opening_range, closing_range)) = snapshot