MudCheckBox: Add state CSS classes for easier testing

[henon]

Description

Testing with checkboxes is a pain. I want to add this to mudcheckbox

                .AddClass($"mud-checkbox-true", BoolValue == true)
                .AddClass($"mud-checkbox-false", BoolValue == false)
                .AddClass($"mud-checkbox-null", BoolValue is null)

otherwise I'd have to compare icon geometry
I did it before but it is far from ideal
the classes have no CSS. but could be used by the user as a selector as well

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.03%. Comparing base (28bc599) to head (1ddfc56).
Report is 49 commits behind head on dev.

❗ Current head 1ddfc56 differs from pull request most recent head ef4ce11. Consider uploading reports for the commit ef4ce11 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8699      +/-   ##
==========================================
+ Coverage   89.82%   90.03%   +0.20%     
==========================================
  Files         412      418       +6     
  Lines       11878    12021     +143     
  Branches     2364     2366       +2     
==========================================
+ Hits        10670    10823     +153     
+ Misses        681      662      -19     
- Partials      527      536       +9     

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