Expose nqp::readlineintfh to perl6-land

[retupmoca]

This gives perl6 code access to the readline library, since the REPL
already uses it.

This could also very easily be written as a module if we don't want this in CORE.setting

[retupmoca]

Note that nqp::readlineintfh does not handle Term::ANSIColor gracefully. It calculates the cursor position by doing a strlen on the prompt, which will be incorrect when using color escape sequences.

If readlineintfh is implemented using readline, the escape sequences can be bracketed with "\x1" and "\x2" to keep them from being counted, but if implemented with linenoise no such workaround is possible.

This means, for example, the current Debugger::UI::CommandLine does not work well with this patch applied, as it uses colored prompts.