Add 'json' encoding to readFile/writeFile

[pfrazee]

Reading and writing objects as json files is a really common pattern, and it's a small-but-notable pain to have to call JSON parse and stringify every time.

Let's add 'json' encoding to readFile and writeFile. If specified, the API will automatically parse (readFile) or stringify (writeFile) the json.

Example:

await beaker.hyperdrive.writeFile('/foo.json', {hello: 'world'}, 'json')
await beaker.hyperdrive.readFile('/foo.json', 'json')

Sugars may clutter the APIs a bit, but developers is users too, and this is an easy DX improvement.

[pfrazee]

Implemented via the pauls-dat-api2 module