[Theme] Base theme setup

[mgadewoll]

Summary

closes #124

[!IMPORTANT] This PR is currently still in Draft mode, as it needs alignment on some package/dependency specific setups/architectural choices.

📖 See additional information here.

QA

• [ ] checkout this PR and verify packages build and run as expected:
  • /eui: run build:workspaces to build workspace dependencies then check that our common development tools/scripts work as expected
    • Storybook: yarn storybook and yarn build-storybook
    • EUI docs: yarn start and yarn build-docs
  • /website: run build:workspaces and then test that EUI+ docs works as expected via yarn start and yarn build

Deployments & theme switcher testing

By default the theme switcher is only enabled for development environments.

Storybook: https://eui.elastic.co/pr_8030/storybook/?path=/story/navigation-euibutton--playground EUI docs: https://eui.elastic.co/pr_8030/ EUI+: https://eui.elastic.co/pr_8030/new-docs/