feat: support serving index.html in middleware mode

[meowtec]

Description

I want to integrate vite (as a koa middleware) to my koa app for just development.

When middlewareMode enabled, vite will not handle the html request. I should serve them like this: Setting Up the Dev Server

I am not working on SSR, and I do not like to serve the html content by myself. Instead, the internal html middleware is enough for me.

It should be able to use indexHtmlMiddleware and spa fallback even when middlewareMode enabled.

Additional context

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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.

[patak-dev]

@meowtec we discussed this with Evan and this feature is approved. About the config API, we think that we should avoid the complexity of an object here.
Let's use proposal at #2871 (comment)

middlewareMode: 'html' | 'ssr' | true

Let's also update the documentation to use 'ssr' instead of true when explaining how to configure SSR.

Thanks for the work on this PR

[patak-dev]

Looks like server.middlewareMode is not documented yet in https://vitejs.dev/config/#server-options. You may also want to include that in this PR or we can do it later. We may also need to think about how to add a test for 'html' mode to avoid future regressions.

[meowtec]

Looks like server.middlewareMode is not documented yet in https://vitejs.dev/config/#server-options. You may also want to include that in this PR or we can do it later. We may also need to think about how to add a test for 'html' mode to avoid future regressions.

document added