Add multiline diagnostic support

[MariaSolOs]

Fixes #279, #268 and #11

A preview of how this looks:

[folke]

Thank you for this PR!

I'll look into it later today, but I think this potentially breaks actions and folds on the different lines. Maybe not though, need to check :)

[MariaSolOs]

Thank you for this PR!

I'll look into it later today, but I think this potentially breaks actions and folds on the different lines. Maybe not though, need to check :)

Thanks to you! There are a couple of design questions in #279 but we should definitely first make sure this doesn’t break anything.

[eatgrass]

@MariaSolOs @folke
please check the screenshot. How about keeping the first line in the entry, it seems more consistent

[folke]

I've made multiline configurable and changed the layout as @eatgrass suggested.
Still have to check if multline possibly breaks something

[folke]

All seems to work as it should. Merged. Thank you!

Edit: made multiline the default

[MariaSolOs]

All seems to work as it should. Merged. Thank you!

Edit: made multiline the default

Thanks to you! I hope people like the new layout as much as I do ☺️

[kting28]

Thanks @MariaSolOs