Update to perspective 3.8.0

[texodus]

This PR updates perspective-summary to be compatible with Perspective 3:

• The API for perspective.worker() is now async, this has been updated in the example projects.
• Example projects updated to use load-order-preserving module syntax.
• Updat call to View::get_config which fails to serialize aggregates (see Fix aggregates field in View::get_config finos/perspective#3053). This has been replaced with HTMLPerspectiveViewerElement::save, which does the same thing, but this can be reverted by upgrading to the soon-to-be-released Perspective 3.8.1. This was caught thanks to @timkpaine's excellent tests for this plugin!
• Updated the package.json definition to pin to Perspective 3.8.0, as well as moving these dependencies to peerDependencies (as the package itself does not import perspective directly).