Serve visual review toolbar script with Webpack

What does this MR do?

What it says on the tin! It also breaks out the webpack configs so that we can run the prod script on just the toolbar entry point for easier iteration and shorter build times.

This MR also refactors the toolbar styles — because I wanted to be sure I did the loaders right 😄.

This should be paired with gitlab-ee!13322, which updates the script to look in the new location.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Closes #61646 (closed)