Create Interactive Sequence Feature

[strangemethod]

Adds the ability to create a sequence of positions that users can click through with prev/next buttons. A sequence can be creating by simply passing in multiple position strings separated by line breaks. This is meant to be a didactic tool for the demonstrate of opening sequences.

[laat]

This is a really cool example of how <chess-board> could be utilized to create a chess opening visializer. However, I feel this patch violates the single responsibility principle, and makes the chess-board do too much.

What if we made a <chess-opening> component that wraps <chess-board> with previous/next buttons that updates <chess-board> with new fen-strings when the user interacts with the component?

I/We should make this component easier to update with MutationObserver on the content (which is the fen-string), so that a <chess-opening> component is not required to set the fen with chessBoard.fen = '';

This PR has some linting issues, before your next Update/PR please run npm test.