Promote CONTROL_FLOW_OP_NAMES to public API

[jakelishman]

Summary

This is generally useful for Qiskit's built-in transpiler passes, and we've had some interest in using it for the same reasons from outside the library. While the control-flow structure from within Rust might change in the future, in the immediate term and from Python space, it feels fair to expose this as public API, since all the information contained in it is certainly already public.

Details and comments

Minor request from @ihincks.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 10623602313

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 32 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.03%) to 89.171%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/accelerate/src/two_qubit_decompose.rs	1	90.82%
crates/qasm2/src/lex.rs	6	91.48%
crates/qasm2/src/parse.rs	24	95.77%

Totals
Change from base Build 10598013395:	-0.03%
Covered Lines:	71623
Relevant Lines:	80321

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💛 - Coveralls

[ihincks]

Thanks @jakelishman !