Experiment with adding undo/redo

[getdave]

As requested in this #6 this PR shows a PoC for adding undo/redo functionality to the standalone editor.

It's far from perfect as the real undo/redo functionality in the Post editor is more sophisticated and accounts for more scenarios.

That said it works at a basic level and should act as a good kicking off point.

[patricklindsay]

I've updated this to allow for a default/fallback set of blocks, so that can reload blocks previously worked on;

export const getBlocks = ( state, defaultBlocks=[] ) => {
	return state.present.blocks || defaultBlocks;
}

  const blocks = useSelect((select) => select("getdavesbe").getBlocks(parse(storedBlocks)));

[getdave]

Yep although grabbing for storage is a side effect so should probably be handled in a control action. Still working that one out.

[getdave]

Still need to fix things so minor undo levels aren't created.

Isolated Block Editor uses different actions to control the behaviour here. I need to look into this

https://github.com/Automattic/isolated-block-editor/blob/82b69eda507554c76b4afecc1f75987955c22e36/src/components/block-editor-contents/index.js#L91-L92