Update minimum Node.js version for Vartheme BS5 to >=18, SASS, PostCSS, and Webpack version to ~5

Problem/Motivation

#3336535: Require Node.js 18 for development in 10.1.x

Proposed resolution

• Align Varbase 10.0.x with the efforts of the Drupal core team for Drupal 10.1.x by updating the minimum Node.js version to >=18.
• Switch to Webpack ~5.0
• Drop Gulp in favor of Webpack, SASS, and PostCSS

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ✅ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes and Update Helper on new feature change/addition
• ✅ Release varbase-10.0.0-alpha1, vartheme_bs5-3.0.0-alpha1

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3362250: Updated minimum Node.js version for Vartheme BS5 to >=18, SASS, PostCSS, and Webpack version to ~5