Add fine-grained control for scrollbar diagnostics

[AaronFeickert]

This PR updates the scrollbar diagnostic setting to provide fine-grained control over which indicators to show, based on severity level. This allows the user to hide lower-severity diagnostics that can otherwise clutter the scrollbar (for example, unused or disabled code).

The options are set such that the existing boolean setting has the same effect: when true all diagnostics are shown, and when false no diagnostics are shown.

Closes #22296.

Release Notes:

• Added fine-grained control of scrollbar diagnostic indicators.

[mikayla-maki]

Could we make this setting backwards compatible? Accept both true/false and the enum?

[AaronFeickert]

@mikayla-maki: I was hoping to do that and tried using deserialization aliasing, but couldn't figure it out. Any thoughts on how?

[AaronFeickert]

@mikayla-maki: this has been updated to make the diagnostic level setting separate, in order to avoid a breaking change.

[bennetbo]

Just passing by, i think @ mikayla-maki actually meant accepting either
true, false (boolean values) and all, error, warning, information, never/off/none (enum values).
You can do this by implementing Deserialize from serde manually instead of using the derive macro. We have an existing setting that is doing the same here

[AaronFeickert]

@bennetbo: Yeah, I assumed that was the intent, and only made the subsequent separate-setting update since I couldn't figure out how to do it.

Thanks for the information! If it's considered worth it to add this complexity, I'll go ahead and give it another shot. I do think it would be a cleaner approach that would make more sense to the user than having two settings with a subtle interaction.

[AaronFeickert]

@mikayla-maki: This is now backwards compatible, as requested!

[bennetbo]

Thanks!