view/svelte: get ssr and client-side hydration working (WIP)

[matthewmueller]

This PR adds replaces the existing view generator with a concept of extendable viewers. Viewers decide how top-level views get rendered.

The goal of this PR is to reach feature parity with the existing view generator. This means:

• Svelte pages and components
• SSR rendering
• DOM hydration
• Bundling assets for production

.... but lays the groundwork for:

• React, HTML, html/template views (and any other viewer that implements the interface)
• Introduces layouts, errors & frame views
• Adds context support
• Support custom transpilations (e.g. markdown to HTML)

This PR has been a long time coming and I'm excited to be nearing the finish line.