Support MDX as a separate language from Markdown

[bates64]

Check for existing issues

• Completed

Describe the feature

Currently, people using Vale on MDX files tell Vale that it's just Markdown.

[formats]
mdx = md

But it isn't! MDX has clear differences from Markdown:

• Comments are different Support custom comment patterns (e.g., MDX) #762
• Vale must ignore lines that are actually JavaScript, i.e. those starting with import
• Vale must ignore JSX expressions e.g. {1 +1}
• Vale must not treat HTML as prose, i.e. <div foo="bar"> should not generate warnings (applies to Markdown also)
  Syntax reference

[discdiver]

+1