Changes

This PR adds a new SSR mode, called split.

Serverless mode creates an entry point for each page, and each entry point is a self-contained application meant to render one single page.

Here's a list of changes:

• added a new split boolean to the build object; by default, the value is false, which means that Astro will keep the current behaviour;
• plugin-ssr.ts will emit two plugins, one to emit SSR server and one for SSR split, and they can't coexist. One is used, and the other won't. The reason why the two plugins live in the same file is that they share the majority of the code;
• in plugin-ssr.ts the code that generates the deserialisation of the manifest was moved into a function, shared between the new plugins;
• the shape of the manifest now changes a bit based on the split value. pageMap is kept for SSR server, while SSR split has a simple pageModule function. Because of that, I created two types: SSRServerManifest and SSRServerlessManifest;
• I created a new option called serverlessEntryPrefix; by default it's entry

Here's an example of the emitted output

├── chunks
│   ├── @my-ssr.6147f6ca.mjs
│   ├── astro.60173704.mjs
│   └── pages
│       ├── _slug_.astro.bbdede9d.mjs
│       ├── about.astro.60a22a11.mjs
│       └── index.astro.23e80271.mjs
├── pages
│   ├── blog
│   │   ├── entry._slug_.astro.mjs
│   │   └── entry.about.astro.mjs
│   └── entry.index.astro.mjs
└── renderers.mjs

Testing

Added some test case

Docs

/cc @withastro/maintainers-docs for feedback!

If you're OK with the APIs, then I will start drafting a PR in the docs repo, because now the payload of astro:build:ssr has an additional property