Update build process to use Bun

[JamesDPC]

It's time to use Bun to improve build times, modernise and simplify.

Goals:

1. Improve (reduce) build time
2. Reduce complexity
3. Move away from various plugins at various levels of maintainership

AC

• Replace requirements with Bun requirements in package.json
• Update scripts in package.json to point at Bun build
• Remove unused and unnecessary configuration
• Add a build.ts to handle the build
• Update src JS and SCSS based on changes to build
• Ensure the documented build entry points continue to operate
• nsw-design-system functionality does not regress
• test components - SlimSelect, MicroModal, VideoPlayer, SideNav for functionality and ensure no regressions
• Update documentation