Add global handling and key themes in config

[jojojames]

This is an example of some additional configuration points I was thinking of. For example, corfu has a defcustom evil-collection-corfu-key-themes, we could plug that in like this as another configuration point. @noctuid @condy0919

[jojojames]

I think the idea of key themes is really nice too and want to support it in the future. Key themes like [debug, find-references, find-usages, find-definition, eval sexp/eval expression/eval buffer/etc] seem too useful to have to require the user to bind it to a leader key. We should at least create some rationale and allow the user to optionally enable/disable it.

In some cases, you actually need key themes. For example corfu has both the regular "emacs" style completion and also tab-and-go (https://github.com/minad/corfu#tab-and-go-completion). A key theme here would allow the user to pick the style they want and then we can bind it for them in evil-collection. Somewhat related to this PR so that's why I'm commenting here.