feat: add support for SSR builds (experimental)

[ElMassimo]

Description 📖

This pull request adds support for SSR builds, using an approach based on this prototype.

The goal is to support use cases such as SSR with Inertia, and pave the way for other cases that benefit from an SSR build.

Background 📜

By detecting whether the ssr flag was set when triggering the build, vite-plugin-ruby will set an SSR entrypoint instead, and skip any output options related to fingerprinting the resulting assets.

This is experimental, these initial conventions are subject to change in patch releases:

• The SSR entrypoint should be a {sourceCodeDir}/ssr/ssr.js file. TypeScript and JSX are supported.
• The output dir will have an -ssr prefix to be separate from client-side builds, for example: public/vite-ssr.

SSR 🚀

The SSR entrypoint can import any universal code (supporting both node.js and browsers), and leverage the ~/ alias as needed.

You can leverage import.meta.env.SSR to execute different code depending on the environment, for example:

if (import.meta.env.SSR)
  require('fs').readFileSync(...) // SSR build runs in node.js
else
  await window.showOpenFilePicker() // Client build runs in the browser

Building in SSR mode

To create an SSR build:

bin/vite build --force -- --ssr

To run the SSR build:

node public/vite-ssr/ssr.js

Upcoming 🔮

EDIT: Shipped the following improvements in #212.

This experimental version does not address build caching, which requires further changes in vite_ruby to ensure the SSR build is not skipped (for now, using the --force flag should do the trick).

It would be desirable to customize the output dir of the SSR build (to be outside of public, for example).

Finally, perhaps adding an ssrBuildEnabled setting to automatically run an SSR build along with the assets:precompile task.