Add support for nonempty variadic axes via + (Issue #378)

[pdcook]

Addresses #378

Can now require that a named or anonymous axes have at least one matching axis by prepending +, as a complement to the zero or more *.

Examples:

Before

def f(x: Float[Array, "batch feature0 *features"]): pass

Now

def f(x: Float[Array, "batch +features"]): pass

Added both positive and negative tests for this in tests/test_array.py.

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(Unrelated) The tests test_numpy_arraylike_work_around_pep484_badness and test_union_of_arraylike did not pass on the main branch even before any changes. This could be a consequence of my environment, but I followed the contributing guide and used uv so I'm unsure of where I could've diverged.

[patrick-kidger]

Thanks for this! I'm afraid I think I prefer to keep this as-is though. The jaxtyping grammar (and implementation!) is already pretty complicated, so I think I'd prefer to keep it simple.