Add ESM build

[Mrtenz]

This package uses a default export, which gets transpiled to exports.default = SafeEventEmitter; by TypeScript. Importing this in an ESM environment does not work however:

// test.mjs
import SafeEventEmitter from '@metamask/safe-event-emitter';

// Logs: `{ default: SafeEventEmitter }`, instead of the expected `SafeEventEmitter`.
console.log(SafeEventEmitter);

Ideally we standardise this package to match the module template, but as a simple solution for now I've added a separate TypeScript config which outputs an ESM version instead. After this change, the console.log above logs the expected value.

[FrederikBolding]

Can we not configure TypeScript to do this? 🤔

[Mrtenz]

Doesn't look like it. There is an outFile setting but it doesn't work for ESM.