feature: add frontmatter for individual slides (slide layout support)

[gpichot]

Hello,

Back Story: I really like Spectacle to write slides as MDX content. I am not really satisfied with the output though as I can't style slides or have different layouts. The Deck templateFn property has a deceiving name :).
To give even more context: I have around 400-500 slides that I use for professional workshops.
Yesterday, I started playing a bit with this repository to add support for custom layouts.

It's a bit similar to what does Slidev: layouts.

It adds support to parse slides starting with layout: as frontmatter content that is passed to the next slide using MDXContent.

---

layout: sidedImage
image: ./assets/provider-to-consumer.svg

---

### Context is provided **by the closest Provider ancestor**

---

The frontmatter content is retrieved to change the layout using the MDXProvider and custom components property:

const componentsMap = {
  wrapper: (props: any) => {
    const { children, frontmatter } = props;
    const { layout = 'default' } = (frontmatter || {}) as { layout: string };
    if (layout && layout in Layouts) {
      const Layout = Layouts[layout as keyof typeof Layouts];
      return <Layout {...frontmatter}>{children}</Layout>;
    }
    return children;
  },
}

export const MarkdownDeck = ({ slides, notes }: MarkdownDeckProps) => {
  return (
    <MDXProvider components={componentsMap}>
      <Deck>
        {slides.map((MDXSlide, i: number) => {
          const MDXNote = notes[i];
          return (
            <Slide key={`slide-${i}`}>
              <MDXSlide />
              <Notes>
                <MDXNote />
              </Notes>
            </Slide>
          );
        })}
      </Deck>
    </MDXProvider>
  );
};

In the end, I wrote around 10 layouts that fit my needs and one of them looks like this when outputted:

Do you think this PR could be something provided by default? (I can also add some configuration to the loader).