Is your feature request related to a problem? Please describe.
It's unclear if/how I can assign built-in markdown rules (e.g. not spell checking within anchor tags[spell-check-here](dont-spellcheck-here) to mdx files

Describe the solution you'd like
if this doesn't already exist, perhaps in the languageSettings setting could add this

languageSettings:
  - languageId: mdx
    inheritLanguageId: markdown

or maybe

languageSettings:
  - languageId: mdx
    extension: 
      -md
      -mdx 

Describe alternatives you've considered
right now I have re-implemented some of the regexes myself