Validation UI tweaks part 2

[XVincentX]

@marcelltoth @XVincentX I think this looks good for now though I don't think the error/warning message needs to be italic. Does it look bad if they're normal font?

I'm still leaning towards a flat list UI similar to what we're doing with the Spectral lint results. We could show the errors/warnings on the specific property lines within the code view too. We can do this later on when Studio supports "request" files though.

We should also consolidate the background colors of the "apply changes" UI in Studio with the validations background. https://github.com/stoplightio/platform-internal/blob/474c91635f611b8de60197f696824f9acad36e7a/packages/studio/src/components/ReadPanel/MarkdownComponents.tsx#L226-L238

Originally posted by @lottamus in #271 (comment)