As pointed out in the
[comment](https://github.com/ruby/reline/pull/413#issuecomment-1168033973),
the code is actually a control sequence and not only for colors.
To make the dialog color APIs safer to use, we should restrict its
usages and extract away the bg/fg concept from the input.
So in this commit, I made these changes:
1. The dialog_*_bg/fg_color APIs only takes and returns color names (symbol):
- :black
- :red
- :green
- :yellow
- :blue
- :magenta
- :cyan
- :white
2. Add additional dialog_*_bg/fg_color_sequence APIs to access the raw code.
https://github.com/ruby/reline/commit/b32a977766
36ca0e58b6