[lldb] Add a setting to change the autosuggestion ANSI escape codes

I'm a big fan of the autosuggestion feature but my terminal/color scheme
doesn't display faint any differently than regular lldb output, which
makes the feature a little confusing. This patch add a setting to change
the autosuggestion ANSI escape codes.

For example, to display the autosuggestion in italic, you can add this
to your ~/.lldbinit

  settings set show-autosuggestion-ansi-prefix ${ansi.italic}
  setting set show-autosuggestion-ansi-suffix ${ansi.normal}

Differential revision: https://reviews.llvm.org/D121064