feat: distinct inline diagnostic theme keys

[Regaez]

What

Adds more specific theme keys to target the inline diagnostic styles:

• hint.diagnostic.inline
• info.diagnostic.inline
• warning.diagnostic.inline
• error.diagnostic.inline

The diagnostic-specific keys are appended to the existing "severity" keys so that it is backwards compatible with all existing themes; they will automatically fallback to use the current styles. Suggestions for alternative theme keys are welcome.

Why

I'm loving the recent addition of the inline diagnostic feature, however, sometimes I have been finding the error messages quite distracting as my helix theme uses quite a lot of red. I'd like to be able to style these so that they are less intrusive and make it easier to read the actual code.

Default

# Theme
"error" = { fg = "red" }

With modified error theme key

While one could simply style the error key in the theme, this has the downside of affecting other parts of the UI (such as the gutter icon and the diagnostic picker). For example, I'd like to dim the text of the inline diagnostic but not dim the gutter icon.

# Theme
"error" = { fg = "red", modifiers = ["italic", "dim"] }

With proposed changes

With the proposed changes, one can now apply distinct styles to just the inline diagnostics without affecting other parts of the UI and can read the code more easily (note, non-dimmed gutter icon):

# Theme
"error" = { fg = "red" }
"error.diagnostic.inline" = { fg = "red", modifiers = ["italic", "dim"] }

TODO

If this change is deemed acceptable, I will update this PR to:

• include documentation of the new theme keys.

[poliorcetics]

This is a backward compatible change, that's nice :)

include documentation of the new theme keys

You should do it already, that way the PR only needs one round of review (hopefully)

[carrascomj]

This closes #11864.