interp: add -s support for read builtin

[davidhulick]

This implements the -s option for the read builtin using the ReadPassword function in the golang.org/x/term package. This is a less featureful (and as a result, simpler) implementation of the -s flag than is present in #865

Like PR865, I am also unsure of how to test the -s flag since it requires interacting with the terminal input.

[mvdan]

Thanks! SGTM, just two nits. I agree testing this is hard and not really worth it, given that it's really three lines of simple code.