Use pure ES modules

[ehmicky]

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

Once #357 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).

At the moment, netlify-redirect-parser is only used by:

• @netlify/config: so support for pure ES modules should be added first there: Use pure ES modules build#3742
• netlify-cli: same thing for Netlify CLI: Use pure ES modules cli#3514

[ehmicky]

Done in #374