[EuiIcon] Move icon assets to a new package

[tkajtoch]

Summary

As part of the EuiIcon refactor initiative, we want to separate SVG icon assets and generated icon components (with svgr) from EuiIcon sources.

While this isn't mandatory from performance or bundler support perspectives, it'll allow us to separate concerns, cache built icons locally between runs, version them separately, create multiple icon bundles, and much more. That being said, this issue shouldn't be worked on until all other issues from the epic are complete.

Acceptance criteria

• A new icons package is created in packages/icons (@elastic/eui-icons)
• All icon assets are moved from packages/eui/src/components/icon/svgs and packages/eui/src/components/icon/assets to the icons package
• The compile-icons script (including related dependencies and configuration files) should be moved to packages/icons and renamed to build
• Built icons should live inside the packages/icons/dist directory
• EuiIcon should be updated to import icons from the new package instead of the assets directory