feat(theme): prepare Autocomplete Classic Theme

[francoischalifour]

This prepares the package and tooling (build, lint, CI) for the upcoming Autocomplete Classic Theme.

There's no need to review the CSS files (except the conventions used) because we'll swap this WIP CSS rules with the official theme next week.

[eunjae-lee]

codesandbox ci is failing here

[francoischalifour]

@eunjae-lee The CodeSandbox CI builds fails because I can't find a way to tell their CI to use the Node version from .nvmrc (Node 14) to use imports with MJS files. We got in touch with them on Twitter.

[eunjae-lee]

Looks good to me 👍

[Haroenv]

if codesandbox is fixed now / later, good for me; otherwise maybe revert to .js and commonjs?

[codesandbox-ci]

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Sandbox	Source
@algolia/js-example	Configuration
@algolia/react-renderer-example	Configuration