feat(capi): add new chewing_bopomofo_String API

[kanru]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.85%. Comparing base (bab0250) to head (abda404).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
capi/src/io.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #661       +/-   ##
===========================================
+ Coverage        0   89.85%   +89.85%     
===========================================
  Files           0       42       +42     
  Lines           0     8789     +8789     
===========================================
+ Hits            0     7897     +7897     
- Misses          0      892      +892     

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