Adds @babel/core to the peer dependencies

[arcanis]

@babel/core is used here:

https://github.com/mdx-js/mdx/blob/master/packages/babel-plugin-apply-mdx-type-props/index.js#L1

That said, I think the dependency could be removed if you were using something like that instead:

const {types: t} = api;

Cf here for an example:

https://github.com/arcanis/babel-plugin-lazy-import/blob/master/index.js#L15

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[johno]

Thanks! Yeah it looks like referencing types from api is the best solution, but for now I'll merge this since it will also work. Thanks for the PR!