For unions, handle string types last

[brentyi]

Examples:

• For arg: Optional[str] and arg: str | None, --arg None will create a None object instead of a string.
  • This was already happening.
• For arg: str | bool, --arg True will create a boolean instead of a string.
  • This is new.
• For arg: str | int, integers will be created when possible instead of strings.
  • This is new.

Questions raised:

• Should we also handle arg: tuple[str, ...] | int?
• How about Optional[list[str]]?

[codecov]

Codecov Report

❌ Patch coverage is 99.76371% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.78%. Comparing base (b6c0cd6) to head (37dc485).
⚠️ Report is 718 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tyro/_docstrings.py	93.61%	3 Missing ⚠️
src/tyro/_argparse_formatter.py	99.70%	1 Missing ⚠️
src/tyro/_fields.py	99.31%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #209      +/-   ##
==========================================
+ Coverage   99.16%   99.78%   +0.62%     
==========================================
  Files          19       28       +9     
  Lines        1552     2302     +750     
==========================================
+ Hits         1539     2297     +758     
+ Misses         13        5       -8     

Flag	Coverage Δ
unittests	99.78% <99.76%> (+0.62%)	⬆️

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