Let Volt render pages, and re-render islands on new props

[dannote]

Companion to elixir-volt/volt#54. It has no effect until a Volt release after 0.20.0; Volt 0.20.0 ignores the new option, and mix ci passes against it.

Changes

• Render function for Volt. Astral.DevServer passes Volt document: {Astral.DevServer, :render_document, [config]}, which renders the HTML a page request is answered with. With Volt's morph setting, Volt's websocket process for each open page renders the page again when sources change and tells the page what changed, instead of the page requesting itself.

• Islands re-render on new props. When the server rendered an island with new props, Volt sets them on the island's element and dispatches volt:element-update. Every adapter handles it through a shared onPropsUpdate helper:

  Framework	How	State
  Vue	props held in a shallowRef	kept
  React	renders again on the same root	kept
  Solid	props read through a signal	kept
  Svelte	unmounts and mounts again	reset

  Props passed to Svelte's mount are not reactive outside a Svelte module, which is why Svelte remounts.

Verification

• mix ci passes: 257 tests, including four new browser tests, one per framework, that update an island's props and check its state.
• A test checks that render_document/2 produces the HTML whose entity tag the served page carries.
• Tried in a real browser on a copy of an Astral site against the Volt branch: renaming a heading updated the page and its Vue Contents island in place, with no reload.

Open points

• Solid only tracks the props an island was mounted with.
• Astral does not turn morph on; sites opt in with config :volt, :server, morph: [preserve: "[data-astral-island]"].