Export mdast type augmentations to be visible to users

[lschmelzeisen]

Is your feature request related to a problem? Please describe.

If a user wants to work directly with the AST output of remark-mdc (instead of only piping it into remark-rehype), there is currently no type safe way to do so. As the type augmentations in src/micromark-extension/types.ts do not show up in the distribution bundle.

Describe the solution you'd like to see

Exporting everything in src/micromark-extension/types.ts from src/index.ts should solve this.