feat: modernize JavaScript and upgrade Node.js

- Upgrade Node.js from 12.16.1 (EOL) to 20.18.0 LTS
- Add engines field to package.json for Node.js >=20.0.0
- Modernize JavaScript code throughout codebase:
  * Convert var declarations to const/let
  * Update function syntax to arrow functions
  * Use template literals instead of string concatenation
  * Modernize jQuery callbacks and async patterns
- Migrate ESLint from legacy .eslintrc to modern flat config
- Update ESLint rules and remove deprecated configurations
- Fix all linting errors and improve code quality
- Modernize JavaScript in HTML templates (helper-map.html, events.html)
- Update Google Analytics gtag function to modern syntax
- Remove unused dependencies and variables

Files updated:
- .nvmrc: Node.js 20.18.0
- package.json: engines field, updated lint scripts
- eslint.config.js: new modern flat config
- app/assets/scripts/main.js: ES6+ syntax, linting fixes
- app/_includes/helper-map.html: modernized embedded JS
- app/_layouts/default.html: modernized gtag function
- gulpfile.js: arrow functions, async/await patterns
- updatedep.js: already modern, formatting fixes

Author: Dale Kunce

.eslintrc

@@ -1 +1 @@
-12.16.1
+20.18.0