Add optional Volt support to phx.new

[dannote]

Adds an opt-in --volt flag to mix phx.new that generates projects using Volt instead of the esbuild and tailwind wrappers.

Volt is an Elixir-native asset pipeline — bundles JavaScript, runs Tailwind CSS, and serves assets in dev with HMR, all through NIFs. No Node.js required.

What changes

• --volt flag for mix phx.new, mix phx.new.web, and the interactive installer
• Generates {:volt, "~> 0.11.0"} with matching config, dev watcher, endpoint plug, layout, formatter, and asset aliases
• Works for single-app, umbrella, and multi-web umbrella projects
• Umbrella projects use named Volt config profiles (config :volt, :my_app_web, [...]), matching the esbuild/tailwind convention
• Rejects conflicting flags (--volt --no-esbuild, --volt --no-tailwind)
• Default output unchanged — Volt is only used when explicitly requested

[SteffenDE]

I don’t think we should add any more options to the generators and rather explore such integrations using igniter. I believe you should be able to use igniter.new to bootstrap a phoenix project and then apply the changes on top.