Add fix for registers A, B and D on the left side of indexed expressions

[craigthomas]

This PR fixes an error with the following types of statements:

LDA  B,X

The symbol B for accumulator B was not being resolved correctly, and would result in a stacktrace of:

AttributeError: 'str' object has no attribute 'is_address_expression'

Registers can now be specified in the left hand side of indexed expressions correctly. Unit tests added to catch condition.

[codecov]

Codecov Report

Merging #56 (46db033) into main (b48f996) will increase coverage by 0.03%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   82.43%   82.47%   +0.03%     
==========================================
  Files          11       11              
  Lines        1526     1529       +3     
==========================================
+ Hits         1258     1261       +3     
  Misses        268      268              

Impacted Files	Coverage Δ
cocoasm/statement.py	90.24% <80.00%> (+0.18%)	⬆️

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