Add shortcuts for beginning / end of composer

This adds Ctrl/Command+Home/End shortcuts for jumping to the start and end of
the composer contents.

Fixes https://github.com/vector-im/riot-web/issues/12438

src/components/views/rooms/BasicMessageComposer.js

@@ -392,6 +392,20 @@ export default class BasicMessageEditor extends React.Component {
         } else if (event.key === Key.ENTER && (event.shiftKey || (IS_MAC && event.altKey))) {
             this._insertText("\n");
             handled = true;
+        // move selection to start of composer
+        } else if (modKey && event.key === Key.HOME) {
+            setSelection(this._editorRef, model, {
+                index: 0,
+                offset: 0,
+            });
+            handled = true;
+        // move selection to end of composer
+        } else if (modKey && event.key === Key.END) {
+            setSelection(this._editorRef, model, {
+                index: model.parts.length - 1,
+                offset: model.parts[model.parts.length - 1].text.length,
+            });
+            handled = true;
         // autocomplete or enter to send below shouldn't have any modifier keys pressed.
         } else {
             const metaOrAltPressed = event.metaKey || event.altKey;