Export Default Parse Rules

[phxtho]

What

Added default parse rule plugins to the export of the package

Why

Whilst converting the prosemirror-math to a remirror extension. I noticed in math-schema.ts that defaultInlineMathParseRules where being destructured into the schema

parseDOM: [{ tag: "math-inline" },...defaultInlineMathParseRules]

Other stuff

1. The remirror extension
2. Judging from the snippets in attempts to support tiptap I think this change will help with adding parse rules to other prosemirror based editors Support Tiptap #27

[benrbray]

Thanks for your contribution (and sorry for the very late reply)! I'm not able to merge your PR since the structure of the project has changed, but I just merged PR #67 which exposes the parse rules.