[Build] added postinstall script to "babylonjs"

[RaananW]

These lines will be presented to anyone installing "bbylonjs" package (umd).

[azure-pipelines]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[azure-pipelines]

Reviewer - this PR has made changes to one or more package.json files.

[azure-pipelines]

Snapshot stored with reference name:
refs/pull/12616/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12616/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12616/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/12616/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/12616/merge
https://gui.babylonjs.com/?snapshot=refs/pull/12616/merge
https://nme.babylonjs.com/?snapshot=refs/pull/12616/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/12616/merge#BCU1XR#0