feat: Mode based theming

[patchunwrap]

This commit adds a generic way of changing the color based on mode from the theme struct. This saves us from having to implement a match statement under every render function and makes the code rather readable whilst adding a lot more customisation to the user.

All theme options now support adding ".insert", ".select" and ".normal" to the key, and will apply those color/style choices directly. Specifying the theme key without those suffixes will be used as a fallback. So current themes should still work as expected.

[patchunwrap]

I'm pushing this up now to mostly get feedback. This approach works and you can test it yourself with the following theme file (github won't let me upload.toml files):
switcher.toml.txt

The following need to be discussed/resolved before merging:

• I need to add the same functionality to highlights.
• What should I/we do with color-modes. Currently my patch ignores that option and I think that's the right decision. I think it's a confusing experience as a user to add .insert to my theme and then have to google the color-modes option when it doesn't work. Additionally it means we would have to specify 3 fields to the theme.get() function, which I'm not keen on?

I'm a new face so I'm happy to bend to the will of whoever maintains/owns this project and I'm also happy to change the approach if needed.

[patchunwrap]

I decided to add a commit that fixes the color-modes functionality and also decided not to try fixing syntax highlighting. Even though syntax highlighting isn't possible I think this is more than enough customization for most people.