`at.set` methods raise "assignment destination is read-only" with `array_api_strict` arrays

[mdhaber]

at.set rnethods return modified copies of read-only NumPy arrays, but they raise for read-only array_api_strict arrays.

import array_api_extra as xpx
import array_api_strict as xp
# import numpy as xp  # NumPy works

x = xp.zeros(())
x = xp.broadcast_to(x, (2,))
x = xpx.at(x)[0].set(1)
# ValueError: assignment destination is read-only

While we're at it, what do you think about making ... a default value for the second argument/index of at?

[lucascolley]

cc @crusaderky