Support inserting arbitrary Dart code; Support disabling default Hash/Eq generation

[fzyzcjy]

Changes

fix (partially) #1850

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 98.86364% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 93.23%. Comparing base (4f9c6a2) to head (c789971).
Report is 40 commits behind head on master.

❗ Current head c789971 differs from pull request most recent head 27acac3. Consider uploading reports for the commit 27acac3 to get more accurate results

Files	Patch %	Lines
.../library/codegen/parser/type_parser/enumeration.rs	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1860      +/-   ##
==========================================
- Coverage   99.26%   93.23%   -6.04%     
==========================================
  Files         366      366              
  Lines       15321    15230      -91     
==========================================
- Hits        15208    14199    -1009     
- Misses        113     1031     +918     

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