Wider integration test coverage

[timfish]

The integration tests should be refactored so that we call the bundler CLIs rather than calling them directly from code. Calling from code has too many limitations.

We should then add teats for:

• Both ESM and CJS config files
• All major bundler versions we support
• New native bundlers - Rolldown, Rspack, etc
• Builds with multiple entry points
• Builds with shared chunks that aren't entry points

[linear]

BUNDLER-111