feat: expose ssrTransform to server

[antfu]

Description

I need this in vitest 👀

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[antfu]

For the context, Vitest is a WIP test running powered by Vite (which you probably already know). It uses server.transformRequest(id, { ssr: true }) to transpile modules and execute the result. It works prefectly fine for vanilla js, but when it comes to Vue (and probably other frameworks too), the ssr: true has also been passed to the plugins which generate SSR version of Vue component. In Vitest, we want to separate those two behaviors into two steps to run the client version of component transform while being able to execute them in SSR.

    // for components like Vue, we want to use the client side
    // plugins but then covert the code to be consumed by the server
    const result = await server.transformRequest(id)
    if (!result)
      return
    return await server.ssrTransform(result.code, result.map, id)

[patak-dev]

Marked the feature as experimental so we can merge it in 2.7, hopefully to be released today 🙌🏼