This PR converts everything (aside from electron) from CommonJS to ESM.
It is needed to reduce the changes that will happen during the migration
to Typescript (as TS does not play nice with CJS).

Basically:
- rewrite `require()` to `import`
- rewrite `module.exports` to `exports`
- introduce `ts-node` to run importers so we can convert them to TS too

Lastly, sorry for this larg-ish PR, not my preference but when I tried
to reduce its scope, I would end up with mixed commons/esm that was even
more tricky to handle.

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Previously, we were using an alpha version of date-fns v2. Now we’re
using the latest stable v2.

Closes #399

This reverts commit ae8ef267.

Revert "Merge pull request #204 from TomAFrench/yarn-workspace-dependencies"

This reverts commit b0ed8d51, reversing
changes made to 5505a143.