add vscode commands for groups and snippets

[Picorims]

Motivation behind these commands:

• The group command allows to navigate in VSCode when multiple groups are setup, as the tab command can't handle multiple groups. The direction is given as a parameter : up, down, left, right.
• The snippet command allows to handle all VSCode snippets, both built-in and from extensions. It allows partial snippet support for any file edited in VSCode (so non supported languages as well), in addition to be able to use any third party snippet. You can navigate a snippet by using "tab" and "system" commands. Using "insert" will break snippet navigation however.

[ryanpeach]

Cool