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

[ihexxa]

• enable collection-variable in pre-request scripting
• merge similar pre-request script test cases into one to improve performance

How to test:

Please run this script and check its output.

const { Variable, VariableList } = require('insomnia-collection');

const v1 = new Variable({key: "myVar1", value: "1"});
const v2 = new Variable({key: "myVar2", value: "2"});

const varList = new VariableList(Variable, [v1]);
varList.add(v2)

console.log(v1.toString());
console.log(varList.all());