Add keystroke for fullscreen mode

[l-lejman]

Suggested merge commit message (convention)

Internal: Add keystroke for fullscreen mode.

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Additional information

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[Dumluregn]

After using the keystroke, it seems like browser steal the focus and the second stroke doesn't toggle editor fullscreen but is handled by the browser. We should add an editor.editing.view.focus(); call either here to the keystroke handler, or directly in the command (I think it makes sense - but then remember to remove this call from button press in UI plugin).

[Dumluregn]

There's a convention to name these entries to sound like human-friendly commands, i.e. start with a verb:

So this should be more like "Toggle fullscreen mode".

[Dumluregn]

I see we use such convention in some places (e.g. for bold plugin) but it's only useful if such const can be used in multiple places. Here we can use fullscreen as a command name directly. Or if we want to have it as a const, then also use it in line 47 where we add the command to the editor.