Fixes #615. I would appreciate double-checking that I didn’t
accidentally delete anything that is important.

Since I’m removing the related API methods, this is technically a
breaking change (even if people would have no reason to remove this
stuff), so we should probably do a major release of the API package.

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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* Make it easier to build the bundle.api.js for the API

* Remove budgetId parameter, move config to top level of API

* that’s a breaking change

* Add support for signing into the server in init()

* Add api.downloadBudget(syncId, { password }) method

* Fix lint errors

* Refactor: extract out getSyncError

* api/download-budget: sync if possible instead of downloading

* Don’t bother with fetching remote files and installing key if the file is local

* *groupId

* FIx lint issues

* Remove extra close+reopen

* Refactor out duplicate load-budget logic

* Trailing commas