Updates for tree shaking

What happened?

Mark doc throws if React is not installed, even if the react renderer is not being used.

The renderers should be exported from a path other than the package’s main entry point so that non-react projects or projects using a custom renderer aren’t required to install React.

To reproduce

1. Install markdoc
2. Try to use the html renderer without React installed.
3. Markdoc throws due to React not being present.

Version

Latest

Additional context

This is especially important so that projects not using React don’t end up with React accidentally bundled into their final output.