feat: add isSsrTargetWebWorker flag to configEnvironment hook

[sapphi-red]

Description

This PR adds isSsrTargetWebWorker flag to configEnvironment hook so that plugins can support ssr.target: 'webworker' correctly.
For example, this configEnvironment hook from sveltejs/vite-plugin-svelte#1020 needs to know if ssr.target: 'webworker' is set.
https://github.com/sveltejs/vite-plugin-svelte/blob/dcec9afa2a9b1bfd5fe8f0767cc871a2b144a9d6/packages/vite-plugin-svelte/src/index.js#L71-L81

https://discord.com/channels/804011606160703521/1304311237332697098/1304350474085859338

[patak-dev]

I think this is ok for v6. Do you know how we can get out of it? Do we need another option so this hack isn't needed?
Edit: oh, IIUC, we could just remove it once v5 support is no longer needed. Talking about that, I wonder if svelte will end up doing a clean cut and require Vite 6 soon after we release

[bluwy]

I'm personally still a bit unsure that resolve.conditions now exclusively only applies to client is a nice behaviour, especially that we try to match compat for the environment API, but I don't think it's worth spiraling into that discussion for now 😅 I'll try to test some things locally on my own.

[bluwy]

Talking about that, I wonder if svelte will end up doing a clean cut and require Vite 6 soon after we release

Dominik is fine with that for vite-plugin-svelte, but we'll likely have to align with sveltekit and the plan could change.