Add theia check:theia-version by default

This will run the Theia version check after yarn install to check
whether multiple versions of @theia/* packages are pulled by direct or
indirect dependencies.

See also eclipse-theia/theia#11483

templates/root-package.json

@@ -2,6 +2,7 @@
   "private": true,
   "scripts": {
     "prepare": "lerna run prepare",
+    "postinstall": "theia check:theia-version",
     "rebuild:browser": "theia rebuild:browser",
     "rebuild:electron": "theia rebuild:electron",
     "start:browser": "yarn rebuild:browser && yarn --cwd browser-app start",