Design high-level versioning for extensions

[tjcouch-sil]

User Story
As an extension developer, I want to be able to update my extension so that other extension developers get latest functionality.

Description
Design what extension versioning should look like. Should the version be put anywhere other than in the manifest? How does versioning interact with sharing types? Should the extension module contain the major version number? Should newer versions of extensions always be loaded regardless of priority in the extension order?

See this discussion for some context.

[irahopkinson]

The usual place to put the version is in package.json so it should at least be there.

[Sebastian-ubs]

I've added a new story for the question about compatibity / dependencies between extensions and the platform and among each other: #664