3673: Run next chunk keyboard shortcut not working

Webkit on Mac translates Cmd+Option+N to Cmd+Option+`, see here:
https://bugs.webkit.org/show_bug.cgi?id=41016

Based on some googling around, it doesn't look like Cmd+Option+N is
actually used by anyone to type in ~, so I register Cmd+Option+` in
addition to Cmd+Option+N as a shortcut for executeNextChunk.

Note that this problem also existed in Chrome/Safari on RStudio Server.