proc-macro: refactor backend handler

[vnghia]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 90.88670% with 37 lines in your changes missing coverage. Please review.

Project coverage is 72.94%. Comparing base (43cdf90) to head (861ab58).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nghe-proc-macro/src/backend/handler.rs	90.90%	19 Missing and 9 partials ⚠️
nghe-proc-macro/src/lib.rs	28.57%	5 Missing ⚠️
nghe-proc-macro/src/backend/mod.rs	95.60%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #564      +/-   ##
==========================================
+ Coverage   72.57%   72.94%   +0.36%     
==========================================
  Files         183      184       +1     
  Lines        4652     4697      +45     
  Branches     4652     4697      +45     
==========================================
+ Hits         3376     3426      +50     
+ Misses        846      839       -7     
- Partials      430      432       +2     

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