feat: add postcss-plugin (facebook#781)

* feat: add postcss-plugin
* fix postcss-plugin readme text
* remove .ts, .tsx from node_modules @stylexjs/open-props globs in postcss.config.js
* do not rely on internal exports path
* fix: remove redundant @layer stylex

apps/nextjs-example/app/stylex.css

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ *
+ */
+
+/**
+ * The @stylex directive is used by the @stylexjs/postcss-plugin.
+ * It is automatically replaced with generated CSS during builds.
+ */
+
+@stylex;