[Visual Refresh] Fix theme package builds

[mgadewoll]

Summary

This PR updates the builds for the new theme packages that ensure Kibana CI runs as expected:

• ensure core-js polyfills are handled as expected
• align eui-theme-borealis package build with eui-theme-common build to ensure both esm and cjs packages are available

QA

• CI builds
• Kibana CI doesn't run into errors of missing modules or import statements (before, after)

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8120/
• Storybook: https://eui.elastic.co/pr_8120/storybook
• New docs: https://eui.elastic.co/pr_8120/new-docs

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 0721f95

[tkajtoch]

The changes look good. I'm approving it now but let me know if there are any additional changes

[mgadewoll]

The changes look good. I'm approving it now but let me know if there are any additional changes

Sure! I realized I forgot to remove "Draft" state 😅
I'll merge this PR first and will need to rebase this one after as they touch similar code around the build script.