Add environment variables to all callbacks

[tobiasgiese]

Having environment variables like AEROSPACE_FOCUSED_WORKSPACE and AEROSPACE_PREV_WORKSPACE in all callbacks would be helpful. Especially while using other tools like sketchybar for instance. Having those envs will reduce aerospace queries and will lower resource consumption.

Additionally, further environment variables would be useful. Some ideas that I have in my mind:

AEROSPACE_FOCUSED_MONITOR_ID: the current focussed monitor id
AEROSPACE_WORKSPACE_MONITOR_ID: the monitor id for the workspace
AEROSPACE_WORKSPACE_WINDOWS: the windows in the workspace

xref #230 (comment)

[nikitabobko]

I just realized that after we fix #278 there won't be a need in environment variables in callbacks

on-focus-changed = '''
    exec-async sketchybar --trigger aerospace_workspace_change "FOCUSED_WORKSPACE=$(list-workspaces --focused)"
'''

It will be as fast as environment variables because everything is executed in process right when the callback is called

But we also need to provide list-workspace --prev-focused as a replacement for AEROSPACE_PREV_WORKSPACE