fix(gatsby-plugin-mdx): allow plugins: [...] in options schema

[mxstbr]

Turns out, there is a use-case for specifying plugins: [...] in gatsby-plugin-mdx as explained in this comment as a workaround for gatsby-browser/node/... support in rehype/markPlugins: #15486 (comment)

This adds plugins to the plugin options schema for gatsby-plugin-mdx! 👍

[wardpeet]

Any tests cases you could add? This won't validate options for plugin-mdx sub plugins, right?

We probably want to "copy" the logic from gatsby in here...

[mxstbr]

This won't validate options for plugin-mdx sub plugins, right?

It will!

rehype/markPlugins won't be validated of course, because we do not look at their gatsby-configs at the moment (hence why you also need to add them to plugins: [], that's what the workaround is for), so there's nothing to validate. But if you add a plugin to plugins: [], it will be validated like any other subplugin 👍

[wardpeet]

I was talking about gatsbyRemarkPlugins & rehype plugins.

[wardpeet]

Can we add a dummy plugin to the test so we actually caught this code change?

[mxstbr]

I was talking about gatsbyRemarkPlugins & rehype plugins.

Now that I look at what I wrote yesterday, I have no idea how I thought that made any sense. Of course the options of those aren't validated. 🤦

Added plugins to the tests, good call!