API for post install scripts

[BryceRussell]

There are some cases where ATP may want to add a post install script to themes. This is useful for running code that only needs to be run once, instead of running it every time a project starts. For example, seeding a content collection with content. ATP could have an API for injecting post install scripts into a theme package.

Related: Modifying a user's package.json

Things to consider:

• Currently, themes do not have to be authored as a package. Requiring a post install script for themes requires themes be authored as packages. That means any ATP themes not published to NPM would have to be authored inside a workspace.

[BryceRussell]

We should avoid using a post install pattern unless it is absolutely necessary, closing for now