Fix onBackspace handling of multiline messages with emojis or mentions

When onBackspace occurs with the cursor at the beginning of a line in a multiline message, it now shortcircuits out to the default backspace handler. This will prevent bespoke handling of emoji and mention deletions to erroneously be executed.

Author: kfc35

ts/components/CompositionInput.dom.tsx

@@ -548,7 +548,7 @@ export function CompositionInput(props: Props): React.ReactElement {
     let blotToDelete = leaf[0];
     const offset = leaf[1];
 
-    if (!blotToDelete) {
+    if (!blotToDelete || offset === 0) {
       return true;
     }