feat: support variable axis

[qwerzl]

This PR adds support for variable font axis. It's quite sad that AFAIK only Google provides this level of configuration.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.50000% with 14 lines in your changes missing coverage. Please review.

Project coverage is 45.91%. Comparing base (d83aa0d) to head (98d3697).

Files	Patch %	Lines
playground/pages/providers/google.vue	0.00%	11 Missing ⚠️
src/module.ts	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   45.25%   45.91%   +0.65%     
==========================================
  Files          56       56              
  Lines        3964     4036      +72     
  Branches      429      450      +21     
==========================================
+ Hits         1794     1853      +59     
- Misses       2130     2145      +15     
+ Partials       40       38       -2     

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[qwerzl]

Hi @danielroe, the type issues are fixed and I've added tests for Recursive. Sorry I didn't see the failed checks yesterday so it took some time.

[danielroe]

Rather than creating a new property (variableAxis) I think we ought to try to use font-variation-settings/variationSettings which could take a string or an object of dimension/value.

We could then use this to render both the font-face and also detect the required variations from CSS directly rather than require it to be configured.

[qwerzl]

@danielroe I've also thought about this, but I'm not sure whether font-variation-settings allows a range of values.

[qwerzl]

Now that unifont is released, I think this PR should be closed.

For fine-grained variable axis control of providers other than google, I'm thinking about a variable axis subsetter built into unifont once klippa supports it.

[danielroe]

we could also open again on unifont - for nuxt/fonts i wanted to think if there was a way to make this zero config but i know it’s taken too long 🙈