create_exercise: Needs to check for a .meta folder

[Stargator]

We are starting to have a need for the create_exercise script to check for files in a .meta directory.

If canonical-data.json, description.md or metadata.yml exist in a dart/exercises/exercise-name/.meta folder, then those should overrule or modify the same content from problem-specifications.

Note: The version number should always be overridden and updated to the latest version.

[Stargator]

This is core requirement for this track to have problem specifications whose test cases are modified to take advantage of Dart features.

[Stargator]

Superseded by #269