feat: remove experimental flag middleware

[ematipico]

Changes

This PR removes the experimental flag for the middleware.

While doing so, I had to change how types work inside the manifest. The middleware is a piece of code that is shared among all pages/entry points, but this doesn't work very well in SSR.

The way the SSR bundling logic works is logic this:

1. the plugin plugin-pages.ts generates a virtual module that looks like this export { middleware, page, renderers } and middleware is imported from a shared chunk.
2. the plugin plugin-ssr tries to import all the virtual modules from the previous plugin, and then tries to take the middleware code and expose it via manifest. Doing so could work better with the current plugin architecture.

So, to ensure that the code generated works with the current runtime, I removed the middleware variable from the manifest. This is safe because the virtual page emitted by plugin-pages.ts also has the middleware code.

This change made me realise that the current architecture is brittle and confusing, although I already know how I can change it. I will follow up with different PRs to make the change less confusing.

Testing

Current tests should all pass.

The examples should be updated after this PR lands.

Docs

N/A

[changeset-bot]

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Latest commit: 14cfed1

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[github-actions]

This PR is blocked because it contains a minor changeset. A reviewer will merge this at the next release if approved.

[matthewp]

Block until 2.6

[sarah11918]

Just a note below re: removing middleware from the list of experimental flags in docs!

[github-actions]

This PR is blocked because it contains a minor changeset. A reviewer will merge this at the next release if approved.

Merge day

merge day