Support Result type when Rust calls Dart back

[fzyzcjy]

Changes

Close #1928

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

All modified and coverable lines are covered by tests ✅

Project coverage is 99.24%. Comparing base (2b87aac) to head (f5fdcac).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1945      +/-   ##
==========================================
+ Coverage   99.23%   99.24%   +0.01%     
==========================================
  Files         371      373       +2     
  Lines       15896    15975      +79     
==========================================
+ Hits        15774    15855      +81     
+ Misses        122      120       -2     

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