i3 compatibility: Support for the `all` criteria

[rhoot]

i3 PR: https://github.com/i3/i3/pull/4460

Description: i3 allows using all as criteria to make a rule match all windows. When attempting to use that criteria in sway it triggers an error:

'for_window [all] inhibit_idle fullscreen': Token 'all' is not recognized

As a workaround I'm using [title=".*"], but it'd be nicer to be able to use [all].