Use Relin::ANSI's buffer instead of calling STDIN.ungetc

[tompng]

Reline::ANSI have a read buffer. We don't need to write read bytes back to STDIN only in a few case.

@buf #=> [A, B, C]
@output << "\e[6n"
stdin.read_partial #=> "DE" + "\e[ROW;COLUMNR" + "FG"
@buf.concat ("DE" + "FG").bytes
@buf #=> [A, B, C, D, E, F, G]

Background

This will reduce the effort (example: kateinoigakukun/irb.wasm#24) of making Reline work with wasm

[ima1zumi]

LGTM!