Add simple wp-env config file

Added a simple wp-env file to import all of the themes into a wp-env
instance.

Added .wp-env.override.json to .gitignore so that those local
customizations (mostly pointing to my local gutenberg instance) will be
safely ignored

.gitignore

@@ -11,6 +11,7 @@ vendor/
 .svnignore
 .svn
 .sandbox-config
+.wp-env.override.json
 
 #Ignoring headstart and language files
 */languages/*

.wp-env.json

@@ -0,0 +1,5 @@
+{
+  "themes": [
+    "."
+  ]
+}