feat: enable headers in pre-request scripting [INS-3379]

[ihexxa]

Changes:

• enabled headers in pre-request scripting
• added unit tests and smoke test

How to test:

Please run this script and check its output.

const { Header, HeaderList } = require('insomnia-collection');

const h1 = new Header({key: "X-Header-1", value: "1"});
const h2 = new Header({key: "X-Header-2", value: "2"});

const headerList = new HeaderList(Header, [h1]);
headerList.add(h2)

console.log(h1.toString());
console.log(headerList.all());