cmd + enter to open files in a new editor group

[rebornix]

Review #702.
We mentioned that users can open files in a new editor group in five ways : https://code.visualstudio.com/docs/editor/codebasics#_side-by-side-editing and the first one is

Ctrl (Mac: 'Cmd') click on a file in the Explorer.

My favorite way is running cmd+enter while navigating files through command palette, which is somewhat similar to the first one. Maybe we can enrich this one.

[gregvanl]

I put Cmd+Enter in Quick Open at the bottom of the list since Quick Open hasn't been discussed yet in the doc and the other 4 gestures are more straightforward.