Add benchmarking for MDX parsing and compiling performance

[johno]

We should have a handful of documents that we benchmark so we know:

• baseline MDX parsing performance
• are notified when performance regresses significantly

We should probably consider also benchmarking rendering for React/Preact/Vue as well.

[cesutherland]

Related — https://github.com/mdx-js/mdx/compare/benchmarks

[cesutherland]

Quick error-handling addition: #1205

The spike looks good to me! It'd be nice to get this into 1.6, in order to have a "benchmark" for v2. I could see the benchmarks themselves living inside individual packages, or at the top level like they are here. Have any opinion on that @johno ?

[wooorm]

closing for now, would be good to revisit if large changes are planned again. But the new RC (see: https://v2.mdxjs.com) is much faster on several benchmarks I performed.