feat(fmt/smi): implement smiles writer

[jnooree]

Checklist

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• Did you also add corresponding tests?

Linked Issues

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• Closes Implement smiles writer #273.

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[codecov]

Codecov Report

Attention: Patch coverage is 94.00000% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 90.67%. Comparing base (a3abb0d) to head (5743382).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
+ Coverage   90.59%   90.67%   +0.08%     
==========================================
  Files          37       37              
  Lines        8431     8636     +205     
==========================================
+ Hits         7638     7831     +193     
- Misses        793      805      +12     

Files	Coverage Δ
include/nuri/fmt/smiles.h	100.00% <ø> (ø)
src/core/molecule.cpp	94.63% <100.00%> (ø)
src/fmt/smiles.cpp	95.23% <93.96%> (-1.05%)	⬇️

... and 2 files with indirect coverage changes