fix: use module augmentation instead of extending config

[brc-dd]

also changes defineConfig to withPwa

[userquin]

@brc-dd vitepress-alpha15 seems to break the pwa: the viteConfig.build.ssr is true inside the transformHead hook, we're getting the api reference from the client build, but it seems to be mutated (shared between Vitepress builds?)

Just forgot it, the hook is always called on SSR: I will change the api to just expose the pwa info with a few more flags

[userquin]

You can merge this PR when you solve the ts problem.

[userquin]

@brc-dd you can check how Vitest do the augmentation here: https://github.com/vitest-dev/vitest/blob/main/packages/vitest/src/config.ts#L3