feat: add standard Fetch API compatible interface

[Lqm1]

Summary

Proposal-oriented PR adding a standard Fetch API compatible fetch() on top of impers' HTTP engine, so impers can be plugged in as a custom fetch for clients TypeScript devs already use (ky, axios, etc.) while keeping impers' browser impersonation / HTTP/2-3 fingerprinting.

Why

ky and axios accept a custom fetch:

import ky from "ky";
import { fetch as impersFetch } from "impers";

const api = ky.create({ fetch: impersFetch });
await api("https://tls.browserleaks.com/json", { impersonate: "chrome124" } as never);

Users keep familiar ergonomics and transparently gain fingerprint spoofing. Even standalone, a Fetch-compatible interface is something TypeScript users expect.

Changes

• src/http/fetch.ts — fetch(input, init?): string | URL | Request input, standard RequestInit, returns a real global Response (.status/.ok/.headers/.text()/.json()/.arrayBuffer()/.clone()/bodyUsed). Network/SSL/timeout/abort errors → TypeError with original error in cause (Fetch semantics); 4xx/5xx returned, not rejected. redirect: follow|manual|error honored, maxRedirects defaults to 20. impers extensions via ImpersRequestInit; fresh Session per call (no cookie/state leakage, shared connection pool).
• src/public.ts — exports fetch + ImpersRequestInit.
• tests/fetch.test.ts — GET/POST, body types, Request input, redirects, errors, body semantics, impers extensions.
• examples/fetch-api.ts — runnable sample.
• tsconfig.json — target ES2020 → ES2022 (required for new TypeError(msg, { cause })).

Limitations

Inherited from the public Response constructor: response.url/.redirected/.type are not populated. Unsupported RequestInit fields silently ignored: credentials, mode, integrity, keepalive, window, duplex, referrerPolicy, referrer.

Test plan

• npm test
• npm run lint
• npm run build

Notes for review

1. Shadowing the global fetch vs. named-only export?
2. ImpersRequestInit extensions flat on RequestInit vs. an impers sub-object?