fix(esm): fix esm/commonjs compatibility issues with custom formatters/snippets

[davidjgoss]

Description

Handle all permutations of exporting a custom formatter or snippet class from userland code, across ESM and CommonJS.

It's ugly but it works and has solid tests, and when we get to a point (probably in another 1-2 major releases) when we can go ESM only then most of the complexity can be removed.

Also, add a couple of missing things to the main export.

Motivation & context

To prevent regression from current functionality as we switch from require to import. This was affecting the pretty formatter with the latest RC.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING document.
• My change needed additional tests
  • I have added tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

[quine-bot]

👋 Figuring out if a PR is useful is hard, hopefully this will help.

• @davidjgoss has been on GitHub since 2013 and in that time has had 165 public PRs merged
• Don't you recognize them? They've been here before 🎉
• Here's a good example of their work: liquibase-linter (Quality control for your Liquibase scripts)
• From looking at their profile, they seem to be good with JavaScript and TypeScript.

Their most recently public accepted PR is: #1895

[coveralls]

Coverage increased (+0.004%) to 98.168% when pulling e7023e0 on fix/deep-imports-and-esm into 3757f3c on main.