feat: Raise an error when an AutoQASM program is nested inside another program

[rmshaffer]

Issue #, if available:
#809

Description of changes:
Raise a NestedMainProgramError if the user attempts to build an AutoQASM program from within the context of another AutoQASM program.

Testing done:
tox passes. Test previously marked xfail is now passing.

Merge Checklist

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General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (8193c7c) to head (a7e7142).

Additional details and impacted files

@@                Coverage Diff                 @@
##           feature/autoqasm      #962   +/-   ##
==================================================
  Coverage            100.00%   100.00%           
==================================================
  Files                   172       172           
  Lines                 10540     10543    +3     
  Branches               2254      2255    +1     
==================================================
+ Hits                  10540     10543    +3     

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