Clip UTF-16 offsets in text for range (#20968)

When launching the Pinyin keyboard, macOS will sometimes try to peek one
character back in the string.

This caused a panic if the preceding character was an emoji. The docs
say
"don't assume the range is valid", so now we don't.

Release Notes:

- (macOS) Fixed a panic when using the Pinyin keyboard with emojis

crates/terminal_view/src/terminal_element.rs

@@ -1001,6 +1001,7 @@ impl InputHandler for TerminalInputHandler {
     fn text_for_range(
         &mut self,
         _: std::ops::Range<usize>,
+        _: &mut Option<std::ops::Range<usize>>,
         _: &mut WindowContext,
     ) -> Option<String> {
         None