Make resolveConfig available in user land

Adds a new `resolveConfig` file to the project root that can be imported as `tailwindcss/resolveConfig` and used to get a fully merged version of your custom config file.

Useful when you want access to your design tokens in JS.

resolveConfig.js

@@ -0,0 +1,6 @@
+const resolveConfigObjects = require('./lib/util/resolveConfig').default
+const defaultConfig = require('./stubs/defaultConfig.stub.js')
+
+module.exports = function resolveConfig(config) {
+  return resolveConfigObjects([config, defaultConfig])
+}