fix: register chroma style once

[aymanbagabas]

don't register chroma style if it's already registered

this could cause a "concurrent map writes" fatal error when used in a
concurrent setting like a bubbletea wish app

[muesli]

I wonder if this is the right place to fix this. Maybe the styles map should be a sync.Map to allow for concurrent access? Alternatively the locking could probably be done in styles.Register and styles.Get itself.

[aymanbagabas]

@muesli alecthomas/chroma#650

[muesli]

Nice, let's see what @alecthomas thinks about this. There might be other areas of chroma that aren't concurrency-safe, so it may or may not make sense to do this. We can still follow this PR otherwise.