fix: remove builder.entireApp

[bluwy]

Description

The idea I had for #16471 (comment). Feel free to let me know too if this is a bad idea

What I'm thinking is that build.entireApp currently has a different meaning:

1. If set by the user or plugins, the CLI is able to dynamically call build() or createBuilder()
2. But from a programmatic API pov, frameworks can't do the same as they need to decide early on which API to use. Perhaps the framework could support both depending on whether the user sets the config, but they can't emulate the behaviour like the CLI.

With this PR, I removed the build.entireApp option entirely and let frameworks manage them themselves.

1. If they only support --app, they should call createBuilder or ensure the vite CLI is called with --app
2. If they only support the normal build API, they should call build() (same as before, not changed)
3. If they support both, users can choose via --app, or frameworks can expose an option to pick between both.

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[patak-dev]

Let's discuss about this one after we merge the PR. We can point to main after that.

[sapphi-red]

I like this.

1. ... ensure the vite CLI is called with --app

I have one question about this one. Is it easily possible to detect whether it was called with --app?

[bluwy]

I guess you could check process.argv.includes('--app') to check that.