feat: add self-provided themes

[jaeheonji]

Now zellij provides its own variety of themes.
The types of themes provided at this point include those that existed in the example folder.

And the example folder will contain only examples in accordance with the intent.

[imsnif]

This is great @jaeheonji !! I'd like to release a version tomorrow morning. Do you think this will be ready in time?

[jaeheonji]

@imsnif
I think I'm almost ready. But only one weird thing is, the current e2e test doesn't pass. Exactly, the timeout occurs, but I don't know why.

[naosense]

last time I also encountered this, the e2e failure disappeared after I recommit the code

[jaeheonji]

Thanks for letting me know. I'll try to make a PR after re-commit.