Add rprompt support for right-side prompt display

[amatsuda]

Summary

This patch adds support for displaying a right-aligned prompt (rprompt) similar to zsh's RPROMPT feature.

• The rprompt is displayed at the right edge of the terminal
• Automatically hides when the input line gets too long (prevents overlap)
• Persists during line editing (backspace, Ctrl-U, etc.) because it's part of Reline's render cycle

Usage

Reline.rprompt = "[%H:%M]"
Reline.readline("> ")

Implementation

• Added rprompt attribute to Reline::Core (reader + setter)
• Pass rprompt to LineEditor in inner_readline
• In LineEditor#render, add rprompt as a segment on the first visible line at [right_col, width, text]
• Uses the same segment-based rendering as dialogs

Motivation

I wanted to have this feature for my app that I'm building on top of Reline, and found it difficult to implement without patching Reline itself.

Testing

Behavior of the RPROMPT feature can be confirmed with something like this.

ruby -rreline -Ilib -e 'Reline.rprompt = "[#{Dir.pwd}]"; print Reline.readline("> ")'

[tompng]

This part adds segment of rpromt to new_lines[idx][2]. This part is OK, but segments added by dialog is now problematic.

Can you also change the dialog segment adding part?

# index 0 is for prompt, index 1 is for line, index 2.. is for dialog
dialog_rows[index + 2] = [x_range.begin, dialog.width, dialog.contents[row - y_range.begin]]

index 2 is now used for rpromt, so dialogs should use index 3..
This is a current restriction of differential calculation assumption: same index represents logically same thing. Sorry for the inconvenience of this restriction.

[amatsuda]

TBH I'm still not confident if I'm doing it right, but anyway I just updated 2 in the code to 3 without understanding anything...

Now this oneliner

ruby -I lib -r reline -e 'Reline.rprompt = "[#{Dir.pwd}]"; Reline.add_dialog_proc(:d, -> { Reline::DialogRenderInfo.new(pos: cursor_pos, contents: %w[A B C]) }); Reline.readline("prompt> ")'

creates this new_lines during render:

[[[0, 8, "prompt> "], [8, 0, ""], [168, 45, "[/Users/a_matsuda/src/github.com/ruby/reline]"]], [nil, nil, nil, nil, [8, 1, "\e[0mA"]], [nil, nil, nil, nil, [8, 1, "\e[0mB"]], [nil, nil, nil, nil, [8, 1, "\e[0mC"]]]

[tompng]

Closing in favor of #879