:recycle: Migrate to ESM modules everywhere in the project, remove redundant code

[nilsnh]

Milestone

This PR migrates the project to use ESM for Netlify functions, removes esbuild dependency and removes redundant code.

Getting there

1. Configured package.json to use type=module .
2. Convert filenames from .mjs to .js everywhere.
3. Update imports.
4. Re-run tests and verify build.

Interesting bits

• EcmaScript modules is quickly becoming the defacto standard for Node.js projects. See: https://nodejs.org/docs/latest-v16.x/api/esm.html#modules-ecmascript-modules
• Until now we have had to distinguish between .js (commonjs) and .mjs (ES) due to limitations in Netlify functions.
• This PR configures Node.js to assume ESM for all .js files and even for the Netlify functions.
• Important note: Some .js files are still old-school scripts that are imported as-is into the browser client-side. For example: locationchange-polyfill.js .

What I need help with

• After having verified that the functionality still works I think I'll just merge this. Do feel free to review the changes after this merge and ask any questions you have.

What's next

After merging, this PR should allow us to:

• More easily develop new features in the future, and face less maintenance churn.

[netlify[bot]]

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