Refactor slate-remark-transformer

[cloverich]

I forked this library ages ago to add some customizations (noted in code comments). In #265 I further trimmed it down dropping unused nodes and its types. Ideally I'd make the following additional changes time permitting:

• Re-work to be a proper mdast-util plugin, rather than a wrapping utility
• Re-work inner switch statement to be a parameter at setup time

i.e. should work more like this:

export const parseMarkdown = (markdown: string): mdast.Root => {
  return fromMarkdown(markdown, {
    mdastExtensions: [mdastSlateTransformer({
      paragraph: (node: Mdast): slate.Node => {}, ...
      code: // ... etc
    })],
  });
};

That would solve the original fork-ing need and better organize the customized code; don't have a good idea of how best to organize the slate / plate / mdast customizations but its in a better state than before, and this change will make it better still.