Add attribute #[frb(dart_async)]

[fzyzcjy]

Changes

Please list issues fixed by this PR here, using format "Fixes #the-issue-number".

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.07%. Comparing base (a44f789) to head (2ffe610).

Files	Patch %	Lines
...src/library/codegen/parser/mir/parser/attribute.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2181      +/-   ##
==========================================
- Coverage   99.07%   99.07%   -0.01%     
==========================================
  Files         487      487              
  Lines       20160    20165       +5     
==========================================
+ Hits        19974    19978       +4     
- Misses        186      187       +1     

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