fix(gatsby-plugin-mdx): Pass node API helpers through mdx-loader to remark plugins (#21211)

* Pass node API helpers through mdx-loader

* Fix parse error

Author: andrewbranch

packages/gatsby-plugin-mdx/gatsby/create-webpack-config.js

@@ -75,6 +75,7 @@ module.exports = (
               loader: path.join(`gatsby-plugin-mdx`, `loaders`, `mdx-loader`),
               options: {
                 cache: cache,
+                actions: actions,
                 ...other,
                 pluginOptions: options,
               },

packages/gatsby-plugin-mdx/loaders/mdx-loader.js

@@ -97,6 +97,7 @@ module.exports = async function(content) {
     cache,
     pathPrefix,
     pluginOptions,
+    ...helpers
   } = getOptions(this)
 
   const options = withDefaultOptions(pluginOptions)
@@ -161,6 +162,7 @@ ${contentWithoutFrontmatter}`
   }
 
   const { rawMDXOutput } = await genMdx({
+    ...helpers,
     isLoader: true,
     options,
     node: { ...mdxNode, rawBody: code },