change prompt to ruby for ruby statements

[ko1]

Instead of adding #ruby on the right of input Ruby expressions,
this patch changes the prompt to the ruby.

This patch can relax issue about #538

[st0012]

It looks like the prompt needs an extra space to change?

Like typing foo's prompt is still (rdbg). And it'll require foo for the prompt to be (ruby)

[ko1]

Now it is intentional because debugger can't understand bre is ruby or debug command.

[st0012]

Why? Since the debugger doesn't autocomplete it, it'll only be a Ruby expression.

[st0012]

The extra space requirement already exists in master, so it's not introduced by the change. But I'm just curious about the distinguishing limitation.

[ko1]

because debugger can't understand the typing bre is ruby or debug command (break).