Support OpenQASM end statements

[danielgaskins]

Summary

• accept OpenQASM 3 end; statements during validation and unrolling
• stop statically unrolled blocks after an unconditional terminator
• preserve conditional terminators inside runtime-dependent branches
• cover global, branch, loop, switch, box, and subroutine behavior

Testing

• pytest -q tests/qasm3/test_end.py
• pytest -q tests
• pytest -q --cov=pyqasm --cov-config=pyproject.toml --cov-report=term --cov-report=xml tests
• pylint src tests bin examples
• isort --check-only src tests bin examples
• black --check --target-version py310 src tests bin examples
• mypy src examples
• qbraid admin headers src tests bin examples --skip=src/pyqasm/_version.py --type=apache
• sphinx-build -W -b html docs/ <output-directory>

Fixes #396

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