feat: Non-blocking stylesheet resolving

[Stranger6667]

Resolves #246

• [ ] Use Atomic for tendrils during doc construction when necessary

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #320 will not alter performance

_{Comparing dd/async-resolver (1afd436) with master (a52171c)}

Summary

✅ 6 untouched benchmarks

[codecov[bot]]

Codecov Report

Attention: 39 lines in your changes are missing coverage. Please review.

Comparison is base (a52171c) 84.25% compared to head (1afd436) 81.84%.

Files	Patch %	Lines
css-inline/src/lib.rs	74.79%	31 Missing :warning:
css-inline/src/resolver.rs	84.61%	8 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
- Coverage   84.25%   81.84%   -2.41%     
==========================================
  Files          19       19              
  Lines        1791     1774      -17     
==========================================
- Hits         1509     1452      -57     
- Misses        282      322      +40     

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