Feature 🚀 : Add a new `Env` type that can be used and extended externally

Adding a new `Env` type in the workers-types package could be beneficial as it could be used/leveraged by users and library/framework authors.

I made this simple POC regarding this: https://github.com/dario-piotrowicz/workers-types-env-poc

(in the POC's `workers-types`'s node_modules directory you can see the type of change I have in mind, only applied to the `experimental` entrypoint there)

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This could be beneficial for uses of `getPlatformProxy` such as [nitro-cloudflare-dev](https://github.com/pi0/nitro-cloudflare-dev) (see https://github.com/pi0/nitro-cloudflare-dev/pull/13)
(cc. @pi0, @Aslemammad)

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We've been also asked about this from the Remix team once. Their reasoning there being that having an `Env` interface somewhere, always guaranteed to exist, allows for libraries to be written with more stable/robust typing instead of having to reply on a global `Env` interface that might or not exist, @pcattori please feel free to add context here regarding this if you want 🙂.

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This needs a bit of discussion though as the introduction of such a type, although not complex would likely not be trivial and the effort of implementing and maintaining such type could outweigh the benefits 🤔