Fix tree sitter chunking (#7417)

Call as bytes before slicing, that way you can take bytes that aren't
aligned to chars. Should technically also be slightly faster since you
don't have to check alignment...

helix-core/src/syntax.rs

@@ -1402,7 +1402,7 @@ impl LanguageLayer {
                 &mut |byte, _| {
                     if byte <= source.len_bytes() {
                         let (chunk, start_byte, _, _) = source.chunk_at_byte(byte);
-                        chunk[byte - start_byte..].as_bytes()
+                        &chunk.as_bytes()[byte - start_byte..]
                     } else {
                         // out of range
                         &[]