Config option to not invert colors for IME prededit text

[xzbdmw]

it takes the surface foreground color for the preedit background and the surface background color for the preedit glyphs, which is not normal in most applications in macos

[shivaduke28]

I am a Japanese user who recently started to use Ghostty on macOS.
I use the terminal with Japanese input frequently and the color inversion tends to feel a bit straining on my eyes.

Many terminal apps use underline for IME preedit texts, and so I would love to see a similar option in Ghostty.
I am using a forked Ghostty that adds a config ime-preedit-style so that users can choose between invert and underline,
and it seems to work well at least on my macOS (like shivaduke28#1).

For reference, I'm attaching screenshots from some terminal applications.
Thanks!

Ghostty

Terminal.app

iTerm2

Rider

VSCode

[pluiedev]

This does make sense to me as a Chinese user at least. Would you mind making a PR for this so we can take a look? Thanks!