Standardised method for executing CI checks on local machine

[tranhl]

There isn't a standardised method for executing CI checks (e.g. tests, linters, etc) from the local machine for this project yet. Setting this up will allow contributors to get faster & more immediate feedback on potential issues with their PRs, which will help reduce friction to the overall contribution process.

[kevgo]

A possible setup matching the current CI tasks:

• npm run lint:dist checks dist/index.js
• npm run lint:eslint already exists
• npm run lint:ec already exists as well
• npm run lint is a new task that runs all three lint tasks. It's implemented like this: run run lint:dist && npm run lint:eslint && npm run lint:ec

This is pretty straightforward and could be all we need here. Some npm plugins make this even easier:

• npm-run-all automatically runs all tasks that start with lint:

  "scripts": {
    // existing scripts here ...
    "lint": "npm-run-all lint:*",

• concurrently executes the three tasks concurrently, for extra snappiness