Allow users to quickly define Rust initialization functions (via #[frb(init)])

[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

All modified and coverable lines are covered by tests ✅

Comparison is base (fcc5740) 99.23% compared to head (c39cee4) 99.23%.
Report is 23 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1580   +/-   ##
=======================================
  Coverage   99.23%   99.23%           
=======================================
  Files         341      341           
  Lines       13276    13295   +19     
=======================================
+ Hits        13174    13193   +19     
  Misses        102      102           

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