Add default_dart_async configuration option

[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 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.12%. Comparing base (5269678) to head (bdc92d8).

Files	Patch %	Lines
frb_codegen/src/binary/commands.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2139      +/-   ##
==========================================
- Coverage   99.12%   99.12%   -0.01%     
==========================================
  Files         485      485              
  Lines       19800    19814      +14     
==========================================
+ Hits        19627    19640      +13     
- Misses        173      174       +1     

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