feat: support iconify icon keys

[tlambert03]

This PR

• takes advantage of superqt iconify support (feat: add QIcon backed by iconify superqt#209) and allows icons to be expressed as iconify keys: https://icon-sets.iconify.design

  Action(id=..., title=..., icon='fa6-solid:arrow-up')

see #132 for follow up with color stuff

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (80c795e) 99.54% compared to head (43b8ddb) 99.54%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #130   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          31       31           
  Lines        1746     1748    +2     
=======================================
+ Hits         1738     1740    +2     
  Misses          8        8           

Files	Coverage Δ
src/app_model/backends/qt/_qaction.py	100.00% <100.00%> (ø)
src/app_model/backends/qt/_util.py	100.00% <100.00%> (ø)
src/app_model/types/_command_rule.py	100.00% <ø> (ø)
src/app_model/types/_icon.py	100.00% <ø> (ø)
src/app_model/types/_menu_rule.py	100.00% <ø> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tlambert03]

cc @jni, @Czaki ... nothing here affects napari, since napari still doesn't use app-model for it's icons. But plugins could conceivable in the future if anyone starts using the commands.icon spec ... this makes it easier for them to pick icons from iconify

[tlambert03]

I pulled off the color spec into #132 ... not entirely sure about that part yet. so this is just iconify now