refactor: convert to lit-element and TypeScript

[web-padawan]

This is a combined PR, it contains code that has already been reviewed in the following PRs:

• BREAKING: convert src and tests to LitElement #144 Convert src and test/unit
• Remove demo and WCT config, update markdown files #145 Remove demo and WCT config, update markdown files
• BREAKING: convert theme and visual tests #146 Convert demo and visual tests
• chore: update index.html, add rollup config [skip ci] #147 update index.html, add rollup config