REPL.prompt!: use lower level than peek to wait on input

[IanButterworth]

Response to #54785 (comment)

I've not been able to reproduce the IOBuffer not marked issue so I'm at the mercy of others testing this.

Requires

• use BufferStream for fake terminal Pkg.jl#3931
• fix var name in reseteof(s::BufferStream) #54859

[Keno]

I think the eof is not required anymore. Unfortunately, I don't know how to reproduce it either, I just was just hitting keys and it died at some point.

[IanButterworth]

I think the actual fix here is to just use peek(term) rather than peek(term, Char) because the latter appears to be broken #54785 (comment) for non-ascii Chars and peek(term) seems to be the simplest public method that gives what we want (a wait for input).