Implement multi-byte FCB declaractions

[craigthomas]

This PR adds the ability to define multiple single byte declarations using a single FCB statement. Previously, all FCB statements only accepted a single byte definition. For example, to define 3 constant bytes, you would have to:

FCB $55
FCB $44
FCB 17

Now, FCB statements allow for multiple declarations per line, separated with a comma. The following would be equivalent to the code above:

FCB $55,$44,17

Unit and integration tests were added to catch new conditions. README.md documentation was updated to reference multi-byte functionality. This PR closes #53

[codecov]

Codecov Report

Merging #78 (b8cf979) into main (0e697ba) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
+ Coverage   94.36%   94.43%   +0.07%     
==========================================
  Files          15       15              
  Lines        1756     1779      +23     
==========================================
+ Hits         1657     1680      +23     
  Misses         99       99              

Impacted Files	Coverage Δ
cocoasm/instruction.py	100.00% <100.00%> (ø)
cocoasm/operands.py	98.29% <100.00%> (+<0.01%)	⬆️
cocoasm/values.py	100.00% <100.00%> (ø)

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