fix: only prevent default on Ctrl key combo (#2558)

Close #2383

The previous check for Ctrl or AltGraph being pressed was too broad and
prevented some default browser shortcuts from working properly, and
created blockers for users on different keyboard layouts

This fixes it by checking specifically for Ctrl keys combo only.

Alt + C is still prevented. 

## Test plan

<!-- Required. See
https://sourcegraph.com/docs/dev/background-information/testing_principles.
-->

1. Try using alt + key combos when focusing in input box - this should
now work
2. Try pressing alt + c when focusing in input box - this should open
the command menu

lib/ui/src/Chat.tsx

@@ -387,7 +387,7 @@ export const Chat: React.FunctionComponent<ChatProps> = ({
             // Checks if the Ctrl key is pressed with a key not in the allow list
             // to avoid triggering default browser shortcuts and bubbling the event.
             const ctrlKeysAllowList = new Set(['a', 'c', 'v', 'x', 'y', 'z'])
-            if ((event.ctrlKey || event.getModifierState('AltGraph')) && !ctrlKeysAllowList.has(event.key)) {
+            if (event.ctrlKey && !ctrlKeysAllowList.has(event.key)) {
                 event.preventDefault()
                 return
             }