This was previously blocked because it ran Kibana's build out of memory. Kibana has since increased its allocated memory and also restructured a lot of the build process, so we should be good to go:

Task: convert src/index.js to typescript; will need to account for the existing definitions in src/index.d.ts as well.

Testing:

• node ./scripts/compile-eui.js (which is a part of yarn build) will create the build artifacts, including eui.d.ts through dtsgenerator + customizations (the follow-ups mentioned in the comment on line 95 are out of scope for this issue)
• when the above successfully generates the intended eui.d.ts file, we'll need to test in Kibana
  • steps are documented in https://github.com/elastic/eui/blob/master/wiki/component-development.md#testing-the-component-with-kibana
  • after building with yarn kbn boostrap --no-validate, test the typescript changes with node scripts/type_check