Add Dark theme support

[dmitrysimkin]

Short description 📝

Add support for Dark mode theme.
When config is set to record dark mode images as well it will do so, record dark and light images

Solution 📦

• Add config
• Updated template to respect dark mode flag

Implementation 👩‍💻👨‍💻

It's a bit raw now, just want to get feedback @BarredEwe and then if fine I can cleanup and add tests

[BarredEwe]

Thanks for pushing this. The direction makes sense, but I see a few blocking issues before this can go in:

1. The dark-mode snapshots currently use the same snapshot name as the light-mode ones. That means they will either overwrite the light reference when recording or compare against the light baseline and fail. The dark pass needs a distinct test/snapshot name, for example with a -dark suffix.

2. This changes the public PrefireSnapshot API in a source-breaking way. The existing closure-based initializers were replaced with value-based ones, so consumer code like PrefireSnapshot({ MyView() }, ...) will stop compiling. If this is not intended as a breaking release, the old overloads should stay and just gain isDark.

3. The non-dark path now forces light mode. That changes existing behavior for previews that explicitly opt into dark mode or depend on the ambient color scheme. I think only the dark pass should force a color scheme; the default pass should preserve current behavior.

I also do not see tests covering the new dark-mode generation path yet. Please add coverage for:

• generating the extra dark snapshot assertion when record_in_dark_mode is enabled
• distinct snapshot naming for dark mode
• preserving the existing public PrefireSnapshot API