chore: clean up pre-request script sdk objects

[ihexxa]

Please review this PR (#7147) firstly as current one is based on it

Changes:

• add simple interpolator for enabling replaceIn methods and cleanups
• add private _index field to follow existing method behavior such as one

How to test

Run this script and check if it runs well:

insomnia.environment.set('name', 'jack');
const welcome = pm.environment.replaceIn("Hello {{name}}.");
console.log(welcome);

[ihexxa]

Close this one as it is too small and a part of this PR: #7172