Mocked node req and res proxies

[pi0]

(see #73 for context)

With unenv we provide almost full Node.js compatibility layer for h3 and nitropack.

However, we are going to remove Node.js dependency step by step and make sure h3 is fully platform agnostic by itself and without or with minimum breaking changes.

For this, I am thinking to introduce a lighter proxy shim for req and res, to use it internally ourselves first and make sure all tests pass with the proxies instead of actually depending on Node.js. It is also useful to develop a web fetch adapter asap (#458)

Nitro mocks might be useful to inspire: https://github.com/unjs/nitro/blob/f71cdebd3003f25c27de6570fb1f9bfab31c67b1/src/runtime/cache.ts#L227

[pi0]

Thinking more, it is too much rework for h3 v1 to reimplement req, res. Using unenv is much much safer.