Add typescript babel plugin (gatsbyjs#173)

Add typescript plugin only for `.ts` and `.tsx` files

We can safely assume that anyone building a typescript gatsby site is using something like: https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-plugin-typescript, so we don't need to force the dependency on anyone not using typescript

packages/gatsby-plugin-mdx/component-with-mdx-scope.js

@@ -3,6 +3,7 @@ const crypto = require("crypto");
 const path = require("path");
 const babel = require("@babel/core");
 const syntaxObjRestSpread = require("@babel/plugin-syntax-object-rest-spread");
+const typescriptPlugin = require("@babel/plugin-transform-typescript");
 const debug = require("debug")("gatsby-mdx:component-with-mdx-scope");
 const slash = require("slash");
 
@@ -18,12 +19,18 @@ module.exports = function componentWithMDXScope(
   if (typeof codeScopeAbsPaths === "string") {
     codeScopeAbsPaths = [codeScopeAbsPaths];
   }
+  const isTS = absWrapperPath.endsWith('.ts');
+  const isTSX = absWrapperPath.endsWith('.tsx');
 
   // hoist pageQuery and any other named exports
   const OGWrapper = fs.readFileSync(absWrapperPath, "utf-8");
   const instance = new BabelPluginPluckExports();
+  const plugins = [instance.plugin, syntaxObjRestSpread];
+  if (isTS || isTSX) {
+    plugins.push([typescriptPlugin, { isTSX }]);
+  }
   babel.transform(OGWrapper, {
-    plugins: [instance.plugin, syntaxObjRestSpread],
+    plugins,
     presets: [require("@babel/preset-react")]
   }).code;