feat(richtext-lexical): mdx support (#9160)

Supports bi-directional import/export between MDX <=> Lexical. JSX will
be mapped to lexical blocks back and forth.

This will allow editing our mdx docs in payload while keeping mdx as the
source of truth

---------

Co-authored-by: Germán Jabloñski <43938777+GermanJablo@users.noreply.github.com>

test/fields/payload-types.ts

@@ -3400,6 +3400,6 @@ export interface Auth {
 
 
 declare module 'payload' {
-  // @ts-ignore 
+  // @ts-ignore
   export interface GeneratedTypes extends Config {}
-}
+}