Add new theme keys for LSP diagnostic tags for dark_plus

[David-Else]

Now #9780 is merged I have added the new theme keys.

Deprecated used to look like:

Now it looks like:

I can't find any code to show unnecessary, but the dim modifier works in general in the theme and does exactly as expected, it just dims the text slightly.

@kirawi Looks good to you?

[the-mikedavis]

For testing unnecessary code you can try:

let x = todo!();
let y = todo!(); // tagged with unnecessary

I believe rust-analyzer also uses it for features that are not enabled, for example the blocks in helix-term/src/application.rs marked with #[cfg(feature = "integration")]

[David-Else]

Cheers, here is an example using #[cfg(feature = "integration")] to show the dimmed effect of unnecessary in dark_plus. It is quite subtle:

and here it is for the default theme:

[kirawi]

Looks good to me