feat: Add default option for fallback theme

[Odilf]

This PR adds a default option that is used in case the system theme is not detected (before it got set to light mode, which I suspect is not usually the desired behaviour). This only applies to Linux since, afaict, macOS and Windows always have some option enabled.

This is useful in desktop-less environments (e.g., a server) and some linux environments (e.g., it was unclear to me how to set it up in Hyprland).

In my case, I was using this plugin on my macOS setup and it worked perfectly, then I tried it on a server to edit files locally and it blasted me with light mode so I had to disable it. Then I installed Linux with Hyprland and it was hard to figure out how to set the xorg setting, so I had to also disable it there. This PR will hopefully help with neovim setups that work consistently across the board! 😁

The default option name might need a bit of bikeshedding. Potential alternatives are default_theme (which I don’t particularly like) and fallback (which I do like). Feel free to use whichever you prefer :)

Also, docs need to be written and perhaps the README needs to be updated. However, I'm not sure exactly where they would go since there is not the usual "configuration" section in neither.

[f-person]

Thanks, nice stuff! I like fallback better too, could you change it? :)

[Odilf]

Done 👍