MudForm: Add Spacing property

[danielchalmers]

Description

Turns the MudForm into a flex display and uses that to add a gap-* class based directly on a new Spacing property.
Resolves #8859.

How Has This Been Tested?

visually

Type 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)
• Documentation (fix or improvement to the website or code docs)

video2.mp4

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 90.18%. Comparing base (28bc599) to head (13870bb).
Report is 163 commits behind head on dev.

❗ Current head 13870bb differs from pull request most recent head e20984b. Consider uploading reports for the commit e20984b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8880      +/-   ##
==========================================
+ Coverage   89.82%   90.18%   +0.35%     
==========================================
  Files         412      423      +11     
  Lines       11878    12275     +397     
  Branches     2364     2407      +43     
==========================================
+ Hits        10670    11070     +400     
+ Misses        681      670      -11     
- Partials      527      535       +8     

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[henon]

LGTM, just a test is missing. That check for the application of the class
As for unifying spacing between the various components I'd say let's do it in a separate PR.

[danielchalmers]

@henon Test added. Do you want me to squeeze in the spacing unification before preview2?

[henon]

Do you want me to squeeze in the spacing unification before preview2?

No hurry. We'll do as many previews as we need.