Add oqpy.gate for gate definitions

[rmshaffer]

This PR implements support for gate definitions (issue #1).

The spec for gate definitions is contained here: https://openqasm.com/language/gates.html

Basic usage is as follows:

prog = oqpy.Program()
q = oqpy.Qubit("q", needs_declaration=False)
with oqpy.gate(prog, q, "rz", [oqpy.AngleVar(name="theta")]) as theta:
    prog.gate(q, "u", theta, 0, 0)
with oqpy.gate(prog, q, "t"):
    prog.gate(q, "rz", oqpy.pi / 4)
prog.gate(oqpy.PhysicalQubits[0], "t")
print(prog.to_qasm())

produces:

OPENQASM 3.0;
gate rz(theta) q {
    u(theta, 0, 0) q;
}
gate t q {
    rz(pi / 4) q;
}
t $0;

[CLAassistant]

All committers have signed the CLA.

[PhilReinhold]

Just the one suggestion, but otherwise looks great!