fix(mdx): add additional typings to mdx api

[lonyele]

Issue: #1210

What I did

Add skipExport and wrapExport typings to mdx api

Context

First of all, thanks for adding some typescript support to mdx. I found that as #1079 was landed, it misses some typings. I personally had a problem with skipExport but I found that wrapExport looks like it needs typing too. Let me know if something needs to be changed

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[lonyele]

hm... both yarn test and yarn test-types works ok on my machine(Mac 10.15.2, node 14.4.0, yarn 1.21.1) Probably help is needed here

[johno]

Looks great, thanks!