feature: add barrier instruction support

[rmshaffer]

Issue #, if available:

Description of changes:

Adds a barrier() instruction so that AutoQASM programs can emit OpenQASM barrier compiler directives, matching the Braket SDK's Circuit.barrier() instruction.

Testing done:

Merge Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a962209) to head (217207b).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #108   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           52        52           
  Lines         2279      2289   +10     
  Branches       266       265    -1     
=========================================
+ Hits          2279      2289   +10     

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[aniksd-braket]

Nit: Since _as_qubit_iterable
can return a list or a GlobalQubitRegister, the return type is correct but could be more precise with Iterable[...] | GlobalQubitRegister

[rmshaffer]

GlobalQubitRegister is already a Iterable[aq_types.QubitIdentifierType], and the caller should only care that what comes back is an Iterable[aq_types.QubitIdentifierType]. I think the extra type hint would be redundant here.

However in general it's confusing that GlobalQubitRegister is both an oqpy.Qubit and an Iterable of qubits - leaking a bit of oqpy abstractions through here, will look for a way to clean this up.

[rmshaffer]

opened #110 to clean up the qubit-related types.