fix(gatsby-plugin-mdx): remark/hypePlugins options schema (#27698)

* fix(gatsby-plugin-mdx): remark/hypePlugins options schema

* Fix typo

* Fix tests

Author: mxstbr

packages/gatsby-plugin-mdx/__tests__/gatsby-node.js

@@ -7,7 +7,7 @@ describe(`pluginOptionsSchema`, () => {
       `"extensions[0]" must be a string`,
       `"extensions[1]" must be a string`,
       `"extensions[2]" must be a string`,
-      `"defaultLayout" must be of type object`,
+      `"defaultLayouts" must be of type object`,
       `"gatsbyRemarkPlugins[0]" does not match any of the allowed types`,
       `"gatsbyRemarkPlugins[1]" does not match any of the allowed types`,
       `"remarkPlugins" must be an array`,
@@ -19,7 +19,7 @@ describe(`pluginOptionsSchema`, () => {
 
     const { errors } = await testPluginOptionsSchema(pluginOptionsSchema, {
       extensions: [1, 2, 3],
-      defaultLayout: `this should be an object`,
+      defaultLayouts: `this should be an object`,
       gatsbyRemarkPlugins: [1, { not: `existing prop` }, `valid one`],
       remarkPlugins: `this should be an array of object`,
       rehypePlugins: `this should be an array of object`,
@@ -33,7 +33,7 @@ describe(`pluginOptionsSchema`, () => {
   it(`should validate the schema`, async () => {
     const { isValid } = await testPluginOptionsSchema(pluginOptionsSchema, {
       extensions: [`.mdx`, `.mdxx`],
-      defaultLayout: {
+      defaultLayouts: {
         posts: `../post-layout.js`,
         default: `../default-layout.js`,
       },

packages/gatsby-plugin-mdx/gatsby-node.js

@@ -6,7 +6,7 @@ const fs = require(`fs`)
 
 const {
   onCreateNode,
-  unstable_shouldOnCreateNode
+  unstable_shouldOnCreateNode,
 } = require(`./gatsby/on-create-node`)
 
 /**
@@ -77,7 +77,7 @@ if (process.env.GATSBY_EXPERIMENTAL_PLUGIN_OPTION_VALIDATION) {
         .description(
           `Configure the file extensions that gatsby-plugin-mdx will process`
         ),
-      defaultLayout: Joi.object({})
+      defaultLayouts: Joi.object({})
         .unknown(true)
         .default({})
         .description(`Set the layout components for MDX source types`),
@@ -94,17 +94,31 @@ if (process.env.GATSBY_EXPERIMENTAL_PLUGIN_OPTION_VALIDATION) {
       remarkPlugins: Joi.array()
         .items(
           Joi.alternatives().try(
-            Joi.object({}).unknown(true),
-            Joi.array().items(Joi.object({}).unknown(true))
+            Joi.alternatives().try(
+              Joi.function(),
+              Joi.object({}).unknown(true),
+              Joi.array().items(
+                Joi.alternatives().try(
+                  Joi.object({}).unknown(true),
+                  Joi.function()
+                )
+              )
+            )
           )
         )
         .default([])
         .description(`Specify remark plugins`),
       rehypePlugins: Joi.array()
         .items(
           Joi.alternatives().try(
+            Joi.function(),
             Joi.object({}).unknown(true),
-            Joi.array().items(Joi.object({}).unknown(true))
+            Joi.array().items(
+              Joi.alternatives().try(
+                Joi.object({}).unknown(true),
+                Joi.function()
+              )
+            )
           )
         )
         .default([])