* Fix the implementation of numpy.array_api.vecdot

See https://data-apis.org/array-api/latest/API_specification/generated/signatures.linear_algebra_functions.vecdot.html

* Use moveaxis + matmul instead of einsum in vecdot

* Fix computation of numpy.array_api.linalg.vector_norm

Various pieces were incorrect due to a lack of complete coverage of this
function in the array API test suite.

* Fix the output dtype nonstandard vector norm()

Previously it would always give float64 because an internal calculation
involved a NumPy scalar and a Python float. The fix is to use a 0-D array
instead of a NumPy scalar so that it type promotes with the float correctly.

Fixes #21083

I don't have a test for this yet because I'm unclear how exactly to test it.

* Clean up the numpy.array_api.linalg.vector_norm code a little bit