Use separate glyphs for hint/info/warning/error diagnostic indicators

[the-mikedavis]

Currently all diagnostics use '●' as an indicator. This can be hard to distinguish at a glance, especially if you are colorblind and/or the theme doesn't distinguish much between warning and error styles for example. We should split these out into separate simple shapes so each severity is easy to recognize.

Also see the discussion in #12060.

The current set of glyphs here is:

• hint ○
• info ●
• warning ▲
• error ■

I'm not super attached to this choice but I've been using it for a while and it seems easy to read.

[koolfy]

As my two cents, carrying over from #12060, my only suggestion would be to consider using a cross "✕" for errors as it's easier to distinguish from other shapes at a glance, in particular with smaller fonts or HIDPI monitors

Especially if we don't make it configurable and thus need a default that will work for most people and most situations

[the-mikedavis]

I will give it a try for a while but I'm wary of ✕ because it can look similar to ASCII x, making it not obvious at-a-glance whether the ✕ is part of an error message or not. Especially if we were to show the diagnostic indicator before end-of-line inline diagnostics:

(I'm pretty sure there's an issue or PR for that but I can't find it)

[peteringram0]

I have been using your branch for the last few weeks and really enjoying this small feature. Please stick with the square error!! Also is there any chance on getting the branch updated against main? Thanks @the-mikedavis

[David-Else]

I will give it a try for a while but I'm wary of ✕ because it can look similar to ASCII x, making it not obvious at-a-glance whether the ✕ is part of an error message or not. Especially if we were to show the diagnostic indicator before end-of-line inline diagnostics:

(I'm pretty sure there's an issue or PR for that but I can't find it)

In this example the x looks really bad, I think the square would be better. Ideally it would be a nerd font, but we don't have #12369 merged yet.