Provide block support for plugin

[Markus Liebelt]

the block editor in Wordpress is now the standard, and it should be easy to support it enough, so that not the classic editor has to be used for PbnViewerJS. The following should be supported:

• Provide PGN and optional FEN in the editor itself.
• Provide in the sidebar all relevant settings: layout, orientation, showCoords, boardWidth, width, ...

There are tutorials available how to do it. See

• block tutorial
• sidebar tutorial