FEAT: generator support normal function

[codingl2k1]

What do these changes do?

• If a normal function is decorated by xo.generator, then the function will be run by to_threads.
• Regardless of whether a function is decorated by xo.generator returns a generator or an async generator or a value, the function should works as expected.

Related issue number

Fixes #xxxx

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a442021) 89.12% compared to head (5d346f6) 89.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   89.12%   89.02%   -0.10%     
==========================================
  Files          48       48              
  Lines        4010     4010              
  Branches      764      764              
==========================================
- Hits         3574     3570       -4     
- Misses        351      355       +4     
  Partials       85       85              

Flag	Coverage Δ
unittests	88.85% <ø> (-0.10%)	⬇️

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