Support more attributes ; Add examples ; Fix generated use statement pointing to self ; Fix linter warning avoid_return_types_on_setters ; Reduce number of generated files

[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 99.32660% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.10%. Comparing base (b68eb1d) to head (1537b49).

Files	Patch %	Lines
...gen/generator/wire/dart/spec_generator/misc/mod.rs	92.85%	1 Missing ⚠️
...rator/wire/rust/spec_generator/misc/ty/delegate.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2140      +/-   ##
==========================================
- Coverage   99.12%   99.10%   -0.02%     
==========================================
  Files         485      486       +1     
  Lines       19814    20042     +228     
==========================================
+ Hits        19640    19862     +222     
- Misses        174      180       +6     

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