Support mount at shadow root

[eagleoflqj]

Fixes #689
Add a parent parameter to mount and unmount, which falls back to document.head when absent.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
css-render	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 26, 2023 2:01pm

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05 🎉

Comparison is base (794d6ac) 91.55% compared to head (d313f83) 91.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1105      +/-   ##
==========================================
+ Coverage   91.55%   91.61%   +0.05%     
==========================================
  Files          10       10              
  Lines         296      298       +2     
  Branches       85       87       +2     
==========================================
+ Hits          271      273       +2     
  Misses         10       10              
  Partials       15       15              

Flag	Coverage Δ
css-render	91.66% <100.00%> (+0.07%)	⬆️
plugin-bem	100.00% <ø> (ø)
vue3-ssr	69.56% <ø> (ø)

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Impacted Files	Coverage Δ
packages/css-render/src/c.ts	100.00% <100.00%> (ø)
packages/css-render/src/mount.ts	87.80% <100.00%> (+0.62%)	⬆️
packages/css-render/src/utils.ts	100.00% <100.00%> (ø)

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[mttzzz]

Hello @07akioni !! Could you please kindly review and expedite the approval of this pull request at your earliest convenience? Thank you!