Partial hydration

[aleclarson]

Feature description

• When a module with .client before its file extension is imported, the importer (and its importers, and so on) is marked as a "server-only" module.
• Any modules marked "server-only" are never loaded on the client, and so those modules cannot produce HTML that relies on hydration.

Implementation details

• The static HTML should include empty <div> elements with data-saus attribute that tells Saus which JS module needs to be loaded for hydration of that subtree.
• Each renderer package (eg: @saus/react) is responsible for hydration of each subtree.
• Each rendered page already has .html.js module. Ensure that module contains a map of data-saus attributes → JS modules, so it can import them and re-render each of the previous subtrees when a user navigates to its page.

[aleclarson]

Putting this in backlog, as it feels more like the responsibility of the UI framework