Add highlighting to indicate which forms have not been changed since their last evaluation

[Olical]

As requested by rwaweber on Discord.

This behaviour can be found in Emacs + CIDER, we should borrow the idea.

It should not be implemented in a single client way, this is a generic feature that should work across all clients. We just need to find a way to track buffer changes to forms, a nice way to highlight those forms in the UI and then integrate our change watching with the eval commands with a kind of middleware.

This might be a great use for expanding the event / hook system.

[practicalli-johnny]

The visual representation for highlighting evaluated code in Emacs is subtle (arguably so subtle that most do not notice this feature exists even though it's on by default)

Hopefully such a feature via Conjure can be configured to be less subtle if desired.

I like the visual design of https://github.com/folke/todo-comments.nvim although this maybe overkill for evaluation markers.

Ideally the evaluated highlight visuals would complement and not conflict with other visuals, especially from LSP clients & Todo comments.