feat: add variant for the ::file-selector-button pseudo-element

[lukewarlow]

This PR adds a variant for the ::file-selector-button pseudo element.

I think I've added to the relevant tests but feel free to point out any others that are required.

Might be worth adding the prefixed ::-webkit-file-upload-button and maybe even ::-ms-browse to get browser support from 78 to 95+, but I'm not sure how to go about doing that. I can see I maybe need to add a function in the helpers file but not sure where it would be called from?

[codecov-commenter]

Codecov Report

Merging #388 (755a4f4) into main (728d187) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #388   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files          56       56           
  Lines        4773     4774    +1     
  Branches     1024     1024           
=======================================
+ Hits         4207     4208    +1     
  Misses        303      303           
  Partials      263      263           

Impacted Files	Coverage Δ
src/config/order.ts	100.00% <ø> (ø)
src/lib/variants/state.ts	100.00% <100.00%> (ø)

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[itpropro]

@voorjaar or @antfu did you have time to review this? I actually have a use case for the pseudo element and want to avoid pulling windi from a PR branch :)