Preserve exact rotations in Clifford+T rebasing

[danielgaskins]

Closes #428

Summary of changes

• Decompose rx, ry, and rz rotations at multiples of π/4 into exact Clifford+T sequences
• Raise RebaseError for unsupported angles instead of silently dropping the gate
• Add QASM 2 and QASM 3 coverage for positive, negative, periodic, zero, unsupported, and conditional rotations

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Reviewing files that changed from the base of the PR and between 036ad4d and 79c4663.

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• src/pyqasm/decomposer.py
• tests/qasm2/test_rotation_gates.py
• tests/qasm3/test_rebase.py

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