Make the compiler aware of custom components about mdx

[wooorm]

There’s no point in supporting the p or wrapper components.

Well, I mean, there are some points. You may know that you are only passing h1 and Alert currently, but it’s not great that you have to pass an ['h1', 'Alert'] array at compile time to do that?

There are some things we can decide at compile time. And some things at runtime. These are different times.
If you move options to the compile time, you lose abilities.
Say, for example, mdx-components is changed. All documents compiled by MDX will need rto be cache-busted.
Or say, someone now passes h1. It won’t work. If we’d do this, we’d need to introduce runtime errors for when people a) pass components that were not listed at compile time, and b) do not pass components that were listed at compile time.

What is the point of this change? If the point is to use a particular complex optimization plugin (a fork of @babel/plugin-transform-react-constant-elements), well, then a plugin could also do what you want?