refactor!: rewrite process as class

[pi0]

We have implemented node:process with named exports to mimic global.process and allow hybrid implementation interface however real process in Node.js extends EventEmitter (process instanceof events.EventEmitter === true) and we cannot have features such as lazy getters or warning on access with just named exports. current process polyfill is also kinda messy!

with This PR:

• process extends EventEmitter
• stdin/stdout/stderr are lazily initialized for perf on first access (named exports from node:process deoptimizes!)
• Dummy getters clearly separated and dummy objects are empty object
• All optional props (that only exist in certain situations) + _* internals marked as undefined to avoid implicit behavior and also reduce bundle
• Internal exposes single UnenvProcess class accepting env, hrtime and nextTick implementation from constructor.
• Main entry handles named exports and provides implementation.
• unenv/polyfill/process extends globalThis.process (if exists) with a Proxy that only fallbacks to unenv one if does not has same prop (relevant to Nitro)

hybrid presets

With this new implementation, hybrid presets such as clouflare can directly pass internal implementation to factory class:

import { Process as UnenvProcess } from "unenv/node/internal/process/process";

const process = new UnenvProcess({
  env: /* custom */,
  hrtime: /* custom */,
  nextTick: /* custom */,
});

[pi0]

merging to validate this method.

@vicb if you have any feedback (or was thinking to revert this altogether, we can discuss, don't worry 👍🏼 )