[chore] Drop webpack 4 support

[yoannmoinet]

This will be merged into the v3 branch, and held until all the breaking changes have been made.

Remove Webpack 4 Support

This PR removes support for Webpack 4, simplifying our codebase and reducing maintenance burden.

Changes

Core Infrastructure

• Removed BundlerFullName type and FULL_NAME_BUNDLERS constant
• Removed fullName and variant fields from bundler types
• Unified webpack handling to use single 'webpack' identifier
• Removed all webpack 4 related dependencies

Build System Updates

• Consolidated buildWithWebpack4/buildWithWebpack5 → buildWithWebpack
• Consolidated configWebpack4/configWebpack5 → configWebpack
• Removed webpack 4 specific test helpers (getWebpackPlugin.ts)

Plugin Updates

• Removed webpack 4 conditional logic

Test Plan

• Unit tests pass
• E2E tests run successfully
• Lint and type checking pass
• No webpack 4 references remain in codebase

Breaking Changes

This is a breaking change for users still on webpack 4. They will need to upgrade to webpack 5 before using future versions of our plugins.