feat: Support theming border width and button font weight.

[DaemonCahill]

This PR adds support for theming field components border width and button font weight.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (dc7000b) to head (873e824).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3020    +/-   ##
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  Coverage   96.32%   96.32%            
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  Files         778      778            
  Lines       21888    21888            
  Branches     7079     7480   +401     
========================================
  Hits        21083    21083            
  Misses        797      797            
  Partials        8        8            

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[jkuelz]

You need to regenerate the design token and theming snapshots to make the tests pass, run the following:
npx jest -u -c jest.integ.config.js src/__integ__/themes.test.ts
npx jest -u -c jest.unit.config.js src/__tests__/snapshot-tests/design-tokens.test.ts

[gethinwebster]

Sorry for re-review, took a look at this again with more of a focus on "we're making more tokens themable" and what the possible unexpected impacts of that are, and I think there are important things that I missed first time round.

(in a nutshell, right now I think theming font-weight-button: normal would have unexpected side-effects, including introducing some potentially serious accessibility concerns)