add /assets folder to plugin build process

[bph]

What problem does this address?

It allows the implementation of a Playground blueprint for a Live preview of the Gutenberg plugin in the repo.
The blueprint was added in #67742

What is your proposed solution?

Add a line to the build process script. That's obviously wrong...

as @t-hamano was wondering about in this comment

[sirreal]

Assets are not part of the plugin zip, the assets would need to be synced to this directory on publish: https://plugins.trac.wordpress.org/browser/gutenberg/assets

[sirreal]

It seems like assets should be synced as part of this flow.

[sirreal]

The contents of https://plugins.trac.wordpress.org/browser/gutenberg/assets should probably also be synced to this repo if we go this route.

[bph]

So the folder /blueprints folder from this repo's /assets folder needs to go into the https://plugins.trac.wordpress.org/browser/gutenberg/assets folder.

Cc: @karmatosed