Set up JavaScript tooling

[nsbgn]

Dependencies: #2

At the moment, plain JavaScript is used with a copy of blockly to create the interface. This was fine for local testing, but if we are to maintain a version of this that is exposed to the outside world, we need to streamline the process (reasons can be reviewed on Blockly's 'get started' page).

The JavaScript tooling ecosystem is fragmented: it's easy to be overwhelmed by the plethora of package managers, build tools, module bundlers, etcetera. I have opted for NPM as our package manager and Parcel as our bundler --- this struck me as easiest. For now, I don't think we need additional build tools. However, I'm open for persuasion towards Gulp, Grunt, Yarn, Webpack, Rollup, etcetera. Additionally, we will use TypeScript to protect our sanity.

Note that you can install npm via conda-forge. I mention this for the benefit of Windows-using colleagues: since we use Conda elsewhere anyway, it's an easy way to install and keep track of the development environment.

[nsbgn]

Commit 4ae9cf8 in the merge-with-interface branch adds the package.json to set up JavaScript tooling.