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

• [x] I have read the CONTRIBUTING doc
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• [x] I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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