Make public the wrapper function for creating Marpit plugin

[yhatt]

Marpit has internal helper function to be helpful creating Marpit plugin. It's exposed in @marp-team/marpit/markdown/marpit_plugin and using in some Marp tools but it is private API yet.
https://github.com/marp-team/marpit/blob/master/src/markdown/marpit_plugin.js

We are considering to expose this interface as Marpit.plugin() static function and named export that can use through export { plugin } from '@marp-team/marpit'.

Our plugin ecosystem has compatible with markdown-it, but it would be good that there is a better interface to access Marpit internals.