feat: supported supports() and layer() in @import at-rule

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

supports supports() and layer() for @import

Breaking Changes

No

Additional Info

TODO:

• fix filter for import
• check supports merging
• check media merging
• check layer merging (with anonymous too)
• avoid usage undefined in favor "" to reduce runtime
• resolve todo in tests
• respect comment after @import key
• check source map generation

[codecov]

Codecov Report

Merging #1377 (f08ee98) into master (7c2cede) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1377      +/-   ##
==========================================
+ Coverage   98.39%   98.48%   +0.09%     
==========================================
  Files          11       12       +1     
  Lines         996     1058      +62     
  Branches      338      361      +23     
==========================================
+ Hits          980     1042      +62     
  Misses         13       13              
  Partials        3        3              

Impacted Files	Coverage Δ
src/runtime/getUrl.js	100.00% <ø> (ø)
src/index.js	100.00% <100.00%> (ø)
src/plugins/postcss-import-parser.js	100.00% <100.00%> (ø)
src/runtime/api.js	100.00% <100.00%> (ø)
src/runtime/noSourceMaps.js	100.00% <100.00%> (ø)
src/runtime/sourceMaps.js	100.00% <100.00%> (ø)
src/utils.js	97.72% <100.00%> (+0.08%)	⬆️

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