Migrate ui/styles to New Platform

[joshdover]

Before we remove ui/public we need to find a new home for the less and scss files in ui/styles and update all imports.

The easy option: src/core/public/styles?

cc @elastic/kibana-design

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[cchaos]

@joshdover So long as they still get imported the same way as before, I don't think we have much opinion on where they "live". The folder you specified seems fine to me.

[flash1293]

@joshdover Is this currently planned for 7.8? I had a problem referencing this from within a new platform stylesheet, not sure whether it's a blocker or possible to work around.

[joshdover]

It is not currently prioritized, since the new webpack build for KP plugins automatically imports styling_constants. Do you need access to the other sheets (eg. bootstrap.less or font_awesome.less)?

[flash1293]

It's about the mixins, so it should work just fine. I will check and report back with a specific error if something breaks.

[spalger]

FYI, this be handled by #73154