Refactor into the recommended boilerplate-card

[ian-craig]

Fixes issue #72 by doing quite a large refactor to port our code into the boilerplate-card template.

Major conceptual changes:

1. Adds a lot of tooling: rollup, typescript, prettier, eslint, etc. These are all very standard in the OSS community, are well documented, and I've found them good to work with as I did the refactor so I see this an improvement.
2. Lit-HTML manages the component lifecycle, and should trigger updates when config or any of the related entities are changed (instead of every time we get a new hass)
3. Rendering is now functional, and we never directly access DOM elements to modify them (the render() function just runs again with new state, and Lit-HTML takes care of applying necessary updates to the DOM). This means all of the logic for showing the state of entities is in the render() function using html template syntax, and so are references to all the click callbacks.

I've also added a Contributing section in the Readme with instructions on how to start a watch/devserver build and attach that to Home Assistant so you don't need to copy files around.

The built JS is now in the /dist directory (git ignore'd), and is split into separate files for the view and editor. This was just the format set up in boilerplate-card, but we could change it to a single output file if you'd prefer. I don't have much insight into the deployment process so @ikaruswill I might need your help on that. I did include (and have tested) their GitHub Actions which build these files and attach them to releases.

[ikaruswill]

LGTM as much as my brain can parse. Let's merge this and patch it along the way if we find any issues. I've decided to overwrite my recent changes after you branched and re-implement them separately.