Move package to dev_dependencies

[goodbaes]

What happened?

The README recommends adding dimengen under dependencies.

Why it should be dev_dependencies

• The library is a code generator only used at build time
  (build_runner build/watch).
• End-users of the app never import dimengen; shipping it in the
  runtime bundle just inflates transitive deps.

Proposed change

dev_dependencies:
  dimengen: ^0.4.0
  build_runner: ^2.4.7