Avoid using wavy text decoration for multiline annotations

[pdawyndt]

Multiline annotations from the linter use the wavy text decoration, which hampers readability of the code fragment. Might be better to use the same background-color annotations as with manual multiline annotations. Using wavy text decoration for inline annotations with background highlighting for multiline annotations also seems to mix better if both are combined in the same code fragment.

[jorg-vr]

I have seen this complaint before and I do agree that this kind of marking is to prominent and distracting

I do not think avoiding the marking only for multiline errors is the correct solution.
Depending on the judge and the code a warning code be created on every line, resulting in equally distracting markings.
I also think that warnings of the same type should always use the same styling, regardless of how many lines they span.

I am looking around what other sytems do, and their wavy underlines are always less prominent.
Eg outlook:

Eg code miror:

I wasn't able to find how outlook renders the wavy underline, but codemirror seems to use svg background images, which is something we might be able to pull of as well

This would also remove some cross browser differences