Use mdxType as prop name (#462)

In some cases it might be desired to access
the type prop. This namespaces it with mdx
to become mdxType. This shouldn't clash with
most components and allows wrapper to more
easily inspect its children's type like you
would with most React elements.

packages/mdx/create-element.js

@@ -1,7 +1,7 @@
 const React = require('react')
 const {useMDXComponents} = require('@mdx-js/tag')
 
-const TYPE_PROP_NAME = '__MDX_TYPE_PLEASE_DO_NOT_USE__'
+const TYPE_PROP_NAME = 'mdxType'
 
 const DEFAULTS = {
   inlineCode: 'code',
@@ -10,12 +10,12 @@ const DEFAULTS = {
 
 const MDXCreateElement = ({
   components: propComponents,
-  __MDX_TYPE_PLEASE_DO_NOT_USE__,
+  mdxType,
   parentName,
   ...etc
 }) => {
   const components = useMDXComponents(propComponents)
-  const type = __MDX_TYPE_PLEASE_DO_NOT_USE__
+  const type = mdxType
   const Component =
     components[`${parentName}.${type}`] ||
     components[type] ||