implemented diff3 functionality

[rodo-gatti]

This PR adds a Rust implementation of diff3.

Fixes #15

[codecov]

Codecov Report

❌ Patch coverage is 89.48571% with 184 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.97%. Comparing base (4eee9ce) to head (83380d1).

Files with missing lines	Patch %	Lines
src/diff3.rs	85.86%	175 Missing and 7 partials ⚠️
tests/integration.rs	99.56%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   86.26%   86.97%   +0.70%     
==========================================
  Files          13       14       +1     
  Lines        6203     7951    +1748     
  Branches      514      602      +88     
==========================================
+ Hits         5351     6915    +1564     
- Misses        851     1027     +176     
- Partials        1        9       +8     

Flag	Coverage Δ
macos_latest	86.94% <89.53%> (+0.72%)	⬆️
ubuntu_latest	87.03% <89.53%> (+0.70%)	⬆️
windows_latest	21.10% <25.27%> (+1.19%)	⬆️

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