feat(nuxt): Detect the server API with @nuxt/kit 4.6

[s1gr1d]

Bumps @nuxt/kit to ^4.6.0 for resolveServerVariant. The Vite server and other non-Nitro builders need this server-API detection.

Looks like a lot of code changes, but most of it is just moved code. Those are the important parts:

• resolveServerApi() replaces getNitroMajorVersion and the local-pkg dependency (we can get the nitro version from there). This is actually the most important and biggest change in the PR.
• On the nuxt server API (no Nitro), the module skips all Nitro setup. With this, Kit prints no NUXT_B8024 warnings
• addServerPlugin → addNitroPlugin. Kit 4.6 deprecates the old name, and the behavior is the same
• all nitro-related code moves to nitroServer.ts

Part of #25072

[s1gr1d]

This is just copy-pasted code from the module.ts file

[s1gr1d]

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