feat: toggle theme mode shortcut

[MayurSMahajan]

Solves: #2304

Feature Preview

2023-07-02.15-39-11.mp4

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PR Checklist

• My code adheres to the AppFlowy Style Guide
• I've listed at least one issue that this PR fixes in the description above.
• I've added a test(s) to validate changes in this PR, or this PR only contains semantic changes.
• All existing tests are passing.

[codecov]

Codecov Report

Merging #2939 (89ff44b) into main (e0ad364) will decrease coverage by 0.02%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main    #2939      +/-   ##
==========================================
- Coverage   66.89%   66.88%   -0.02%     
==========================================
  Files         415      415              
  Lines       19767    19777      +10     
==========================================
+ Hits        13223    13227       +4     
- Misses       6544     6550       +6     

Flag	Coverage Δ
appflowy_flutter_integrateion_test	64.48% <40.00%> (-0.09%)	⬇️
appflowy_flutter_unit_test	12.51% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
..._flutter/lib/workspace/application/appearance.dart	81.38% <0.00%> (-1.77%)	⬇️
...utter/lib/workspace/presentation/home/hotkeys.dart	66.66% <66.66%> (ø)