feat: ssrBuild flag in config env

[patak-dev]

Fixes #8835

Description

Adds a flag to apply conditional configuration depending on SSR build. This doesn't apply for SSR dev, as the config is shared for it.

Should we pass the value of config.build.ssr instead, so the user can also config depending on the SSR entry?
Edit: Blu said yes, it is now implemented as string | boolean 👍🏼

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

[netlify]

✅ Deploy Preview for vite-docs-main ready!

Name	Link
🔨 Latest commit	15b264b
🔍 Latest deploy log	https://app.netlify.com/sites/vite-docs-main/deploys/62c125aa5b24960008c33334
😎 Deploy Preview	https://deploy-preview-8863--vite-docs-main.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[patak-dev]

Alternative for the API, defineConfig could receive a PartiallyResolvedConfig as the env. So instead of ssrBuild, it will be:

export default defineConfig(({ command, mode, build: { ssr }}) => {
  // ...
}

The issue that I see is that people without TS may be confused by using build as a flag as:
build ? ... : ..., or be confused if they use ssr alone which may seem to imply is also true for dev SSR.

So I prefer we keep the current ssrBuild API and a flat env.

[patak-dev]

Modified the PR so ssrBuild is boolean only as decided in the last team meeting.

[patak-dev]

There seems to be little usage for this feature. We opened a discussion to check if we should stabilize or deprecate it in Vite 5:

• Stabilizing `ConfigEnv` `ssrBuild` #13809

Are you currently using this feature @Kingdutch?