feat!: add isPreview to ConfigEnv and resolveConfig

[bluwy]

Description

Add isPreview to ConfigEnv. Supersedes #12298

I made a very direct breaking change to ssrBuild (renamed to isSsrBuild) and we could decide if it's a good idea or not 😬

This will break loadConfigForFile mostly and its first parameter accepts a ConfigEnv. However, I don't think it's hard to fix it.

Additional context

• Discussion to stabilize ssrBuild: Stabilizing `ConfigEnv` `ssrBuild` #13809
  As the discussion mentioned, ssrBuild is not widely used.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[patak-dev]

For reference, we also use ssr instead of isSsr in other places. Maybe because of the sSs looking a bit strange there?
That being said, I'm good with the rename. I don't think this will be an issue when upgrading.

[bluwy]

Yeah I also find the sSs a bit odd too, but with isPreview beside it I think it looks more consistent now.

[bluwy]

Before merging, make sure to set the co-author!

Co-authored-by: Romuald Brillout <git@brillout.com>

[brillout]

Very neat. (I've used many ugly hacks in the past to workaround this.)

set the co-author!

No need to :)

[sapphi-red]

What is the plan for avoiding #8912? (https://discord.com/channels/804011606160703521/831456449632534538/993429210867707914)
For example, are we going to say "call loadConfigFromFile for each build separately"?
Otherwise, I guess vitepress won't be able to pass the correct value.

[bluwy]

The idea I'm thinking is that, if you don't know or can't be sure what it is, you can simply set false (we do the same for vitestSetup.ts in this PR). Maybe it makes more sense to allow undefined to convey the "i don't know part"?

[bluwy]

I've pushed a commit that makes them undefined-able

[patak-dev]

I was thinking we could keep a ssrBuild getter with a warning to guide folks in the transition, but given the feature is rarely used I think we could merge this one with out that.