root to defaultID

[chhsiao1981]

1. getRootID => getDefaultID.
2. add setDefaultID in index.ts.
3. have getRootID with deprecation warning.
4. update README.md

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.63%. Comparing base (fda5d08) to head (469de39).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/states.ts	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
- Coverage   90.88%   90.63%   -0.26%     
==========================================
  Files          20       20              
  Lines         406      395      -11     
  Branches       52       48       -4     
==========================================
- Hits          369      358      -11     
  Misses         37       37              

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