feat(js): add ImpitRequest class for header inspection before sending

[barjin]

Adds the ability to inspect final merged headers (including browser fingerprint headers) before sending a request. This enables request signing (AWS S3, custom APIs), debugging, and dynamic logic based on generated headers.

Example usage:

const impit = new Impit({ browser: 'chrome' });

// Create request and inspect final headers
const request = new impit.Request('https://example.com', {
  headers: { 'X-Custom': 'value' }
});

// Headers include fingerprint headers (User-Agent, sec-ch-ua, etc.)
console.log([...request.headers.entries()]);

// Use for signing, then send
const signature = createSignature(request.headers);
const signedRequest = new impit.Request(request, {
  headers: { ...Object.fromEntries(request.headers), Authorization: signature }
});

await impit.fetch(signedRequest);

Also adds impit.getRequestHeaders(url, init) for simpler header inspection without creating a request object.

Closes #227