fix: add $app and $env to optimizeDeps.exclude

[dominikg]

to allow prebundling for libraries using them

see https://discord.com/channels/457912077277855764/1024975960564580383

note that optimizeDeps.exclude automatically excludes subpaths too, implementation in vite looks like this:
i === exclude || i.startsWith(exclude+'/')

cc @bluwy @babichjacob

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[dominikg]

this PR may improve the situation for vitest unit tests using $app or $env too

[dummdidumm]

It doesn't fix #6259, but I think this solves #1485 (not completely but) for people who want to consume libraries using SvelteKit-specific stuff.