Add parser support for "extern port" and "extern frame" statements

[hodgestar]

OpenPulse specifies that ports and frame may declared via "extern port" and "extern frame" statements (see https://openqasm.com/language/openpulse.html#ports and https://openqasm.com/language/openpulse.html#frames).

However OpenQASM only supports extern functions (not identifiers) so both of these statements are currently unsupported and raise parsing errors.

The OpenPulse examples also use "extern port". See https://openqasm.com/language/pulses.html#inline-calibration-blocks and https://openqasm.com/language/pulses.html#inline-calibration-blocks for relevant examples.

This merge request adds support for "extern port" and "extern frame" statements. Its intended to be a minimal parse update to match the current OpenPulse specification.

This issue was reported in openqasm/openqasm#577 and I've also encountered it myself in Zurich Instruments' OpenPulse compiler.

[hodgestar]

I'll make a separate PR to update the tests to not require Python 3.7.

[hodgestar]

Separate PR to fix the CI failures is #38.

[hodgestar]

Ready for final review / merging.

[braised-babbage]

Maybe it's overkill for this MR, but if we ever move towards allowing external linkage with other types it might make sense to have one statement type for this

externDataStatement: EXTERN (scalarType | arrayType) Identifier SEMICOLON;

Then the AST would be something like

@dataclass
class ExternDataStatement(QASMNode):
    """
    Node representing an extern data declaration.
    """

    name: Identifier
    type_: ClassicalType

[braised-babbage]

I'm OK with this as is -- it solves the problem of bringing the reference parser in line with the spec. Re: more general usage of extern, I am curious what the TSC thinks about the plausibility of this. As a reference point, C supports extern linkage for both data and functions, though it's less clear to me whether the data part is a good idea. Not suggesting that we should be taking the cue from C in the OpenPulse context.