Support custom gate defintions in QASM parser

[daxfohl]

Resolves #3560

• Tests verify parsed vs expected, for unparameterized, parameterized, and for broadcasted qregs.
• They also use assert_qiskit_parsed_qasm_consistent_with_unitary to ensure consistency of parsed circuit (and sanity check that the provided QASM definition is actually valid).
• Several tests for invalid gate definitions or invocations and verify similar error message for Cirq and qiskit parsers.
• One odd instance where qiskit does not throw an error on an invalid QASM circuit, even though simulating it in qiskit fails; chalking it up to a qiskit bug.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.87%. Comparing base (0361a81) to head (d29c116).
Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6917    +/-   ##
========================================
  Coverage   97.87%   97.87%            
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  Files        1084     1084            
  Lines       94420    94791   +371     
========================================
+ Hits        92409    92780   +371     
  Misses       2011     2011            

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[dstrain115]

This is a really cool enhancement to qasm parsing!!!
Just some doc comments, and then I think we are good to go.