docs(guides): add resolve.extensions to typescript.md (#1341)

Otherwise it's not possible to resolve imports in the entry file from other .ts or .tsx files

Author: Achim Krüger

content/guides/typescript.md

@@ -62,6 +62,9 @@ module.exports = {
      }
    ]
  },
+ resolve: {
+   extensions: [".tsx", ".ts", ".js"]
+ },
  output: {
    filename: 'bundle.js',
    path: __dirname