Fix unicode bug in LanguageLayer

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
                         &[]