Restructure codebase into multiple files

[thybag]

Based off of https://github.com/nathan-gs/ha-map-card/pull/47

I've had a go at reorganising the codebase to make it a bit more manageable as it grows. Effectively separating the classes to their own files and adding a build system. Initially organised around classes, but util will likely makes more sense to separate into discrete methods at some point, but though best to start simple(ish).

• Split classes to their own files.
• Add NPM to manage dependences (leaflet/lit now built)
• Setup rollup to handle build/watch
• Updated HACs file to reference built copy in dist.

Some of the class types are a little arbitrary, but hopefully this arrangement makes rough sense.

To get going

• npm install
• npm run watch - to build dist automatically when files change or npm run build - for a one off build.

[nathan-gs]

Looks good, except for the 2 minor comments

[nathan-gs]

Thank you @thybag!