Implement the PLANE mnemonic

[craigthomas]

This PR adds the PLANE mnemonic to the assembler. Integration tests updated to match new functionality. This PR closes #7

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.25%. Comparing base (7643280) to head (657a873).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   80.00%   83.25%   +3.25%     
==========================================
  Files           3        3              
  Lines         215      215              
==========================================
+ Hits          172      179       +7     
+ Misses         43       36       -7     

Files	Coverage Δ
chip8asm/statement.py	90.00% <ø> (+5.00%)	⬆️

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