Enable writing module code directly to file

[asvarga-sedaro]

Summary

Enables generating code directly into files:

with open('code.py', 'w') as f:
    module.write_code(f)

rather than

code = module.code
with open('code.py', 'w') as f:
    f.write(code)

It does this with a subclass of CodegenState that avoids building in memory:

• the tokens: List[str] field of CodegenState
• the "".join(state.tokens) string of code

Test Plan

• hatch run test passes
• we could add some tests that compare the contents of written files written directly vs indirectly

Notes

• CodegenState.pop_trailing_newline() isn't supported because the token is already written to the file. this is fixable if desired.

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[codecov]

Codecov Report

Attention: Patch coverage is 59.09091% with 9 lines in your changes missing coverage. Please review.

Project coverage is 91.23%. Comparing base (72701e4) to head (8767302).
Report is 2 commits behind head on main.

Files	Patch %	Lines
libcst/_nodes/internal.py	60.00%	6 Missing ⚠️
libcst/_nodes/module.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1173      +/-   ##
==========================================
- Coverage   91.26%   91.23%   -0.03%     
==========================================
  Files         261      261              
  Lines       26877    26898      +21     
==========================================
+ Hits        24529    24541      +12     
- Misses       2348     2357       +9     

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[zsol]

Please write tests exercising the new code

[zsol]

Seems like the class hierarchy should be the other way around. CodegenState should specialize CodegenWriter.

Then we wouldn't have to have an awkward default value for writer.

[zsol]

this is incorrect, code that doesn't end in a trailing newline won't be serialized properly (for example the python module "" will be serialized as "\n")