`commands.diff`: request for new `kind`: `file`

[GitMensch]

The "kind": "file" would feature two options: from and to (or diffFrom and diffTo).

both could contain:

• enum value previousEditor (or previous) selects the one that was previously opened
• enum value currentEditor (or current) selects the one that is currently opened
• a string, which would point to a filename (whih woul feature variable substitution, so can be explicit specified an/or selected via input vaiables)

the two options allow to specify which is on the left and which o the right side

As commands.diff was not published yet it would also be possible to drop the kind and provide enum values to select what it currently does as from / to.