Ensure no escape sequence before printing prompt

[tompng]

Some test (readline-ext, bundler) expects output of Readline.readline("prompt>") to match /\Aprompt>/.

Readline prints "\e[?2004h" (enable bracketed-paste) before prompt if STDIN and STDOUT is both tty.

$ ruby -rreadline -e "Readline.readline 'prompt>'" | ruby -e "p STDIN.read"
"\e[?2004hprompt>a\b\e[Kb\b\e[Kc\n\e[?2004l\r"

Readline does not print escape sequence before prompt if STDIN or STDOUT is not a tty. But Reline prints it.

$ printf "a\bb\bc\n" | ruby -rreadline -e "Readline.readline 'prompt>'" | ruby -e "p STDIN.read"
"prompt>a\b\e[Kb\b\e[Kc\n"

This pull request fixes Reline to align with Readline.

Background

Ruby ci was failing. https://github.com/ruby/ruby/actions/runs/9352049059/job/25739247917
This pull request fixes the problem of Reline side. Unfortunately, test still fails. I'll also open a pull request to Rubygems to relax the test assertion.

[k0kubun]

Now that the rubygems PR has been merged, let's use this to pass the CI without reverting the previous change. I reverted the revert now, so let me merge this.