Use pure ES modules

[ehmicky]

See background at https://github.com/netlify/team-dev/issues/36

Once #3741 is done and released, we should use pure ES modules and make a major release.

This is more than just switching from CommonJS to import/export. See this list for other changes which might be involved. This should be broken in many PRs, as much as possible, to lower the risk. Also, non-breaking changes (such as adding file extensions in imports, or loading JSON files differently) should be done before the breaking changes (such as using import/export statements).

This applies to all packages of this monorepo, including @netlify/build and @netlify/config.

This should only be done after Netlify CLI has done it first (netlify/cli#3514).

This would not break sites because:

• The buildbot is running @netlify/build as a binary.
• The CLI would support pure ES modules.
• CommonJS plugins would still be supported. They be detected as CommonJS by import().