[RFC] Add `melos` to help manage Flutter projects and restructure the template

[manh-t]

Why

In order to maintain the template easier with some upcoming projects (example projects, templated project, project-to-work, etc.), I would like to add melos to manage our repository as mono-repos.

We have practiced with Melos before. Here are some repos that applying Melos:

• https://github.com/nimblehq/flutter-showcases
• https://github.com/nimblehq/flutter-monorepo-example
• We can refer to Riverpod for more examples: https://github.com/rrousselGit/river_pod

If you agree with this, please give a 👍 , otherwise 👎 .

Who Benefits?

Developers

[manh-t]

This issue is invalid since we decide to go with #58