chore: add ESLint configuration and fix lint errors

[usernane]

Problem

No linting configuration exists. Code style is inconsistent and potential bugs (like the wriable typo) go undetected.

Suggestion

Add ESLint with a standard configuration.

Setup

{
  "devDependencies": {
    "eslint": "^8.0.0"
  },
  "scripts": {
    "lint": "eslint AJAXRequest.js",
    "lint:fix": "eslint AJAXRequest.js --fix"
  }
}

Configuration

Create .eslintrc.json with rules appropriate for browser JavaScript:

• no-undef to catch misspelled variables
• no-unused-vars to detect dead code
• consistent-return for functions that sometimes return and sometimes do not
• eqeqeq to enforce strict equality

Expected Outcome

• All existing lint errors fixed
• Lint runs as part of CI pipeline