Backported Webpack Config for Flarum JS/TS Compilation

This fork of the Flarum Webpack config backports key changes from Flarum 2.x to 1.x.

Background

In Flarum 2.x, the JavaScript compilation process was streamlined by removing unnecessary ES5 polyfills, resulting in a reduced bundle size and modernized ES6 output. These changes can be found in the original PR #3699.

Key Changes

• Outputs ES6 JavaScript instead of ES5.
• Reduces bundle size by excluding unnecessary ES5 polyfills.
• Requires recompilation of all extension JS source code with the updated Webpack config.

These changes aim to future-proof the codebase and enhance performance by utilizing modern JavaScript features.