Apply theme_index from configuration on startup

[iainh]

Currently the theme_index is always set to 0 on startup, regardless
of what is in the configuration file. theme_index is loaded on startup
however so apply it when creating the editor component.

[mcobzarenco]

Thanks for the PR & sorry for being so slow! The "settings file" is really half-baked, as you can see there's not a lot to configure right now. I am working on changing this in #29 and making more things configurable (e.g. what tree sitter parsers are support, modes, key bindings etc.). In the meantime, we should get this fix in.

[mcobzarenco]

One thing I thought about is some sort of validation that theme_index is not out of bounds -- as that would just panic, saying "internal zee error", which isn't in this case. But given settings are being rewritten, I think it's fine as is.

[iainh]

I actually have a working branch myself where I've been playing around with the highlight-with-queries branch changes as well so I completely understand that anything in settings.toml is ultimately temporary. Thanks for merging this one though, hopefully it is useful to someone in the interim.