Fix parsing of unrecognized attributes

[bbrk24]

Fixes #310

Overview

If Mockingbird encounters an unrecognized attribute, it gets stuck in an infinite loop. Although this was first noticed with @Sendable, the introduction of global actors means that there may be arbitrary attributes which are unknowable to the generator.

This PR fixes the behavior when encountering an unrecognized attribute, so that the generator no longer gets stuck.

Test Plan

I added a test case which uses @NotARealAttribute to test that the parser skips over unrecognized attributes.