Add formatter.version Parameter to pubspec.yaml

[GiancarloCante]

Similar to how we can specify page_width in the formatter configuration:

formatter:
  page_width: 123

It would be useful to allow specifying the formatter version explicitly:

formatter:
  version: 3.7.0

Why?

There are many public packages that cannot update their minimum SDK version to Dart 3.7.0 due to compatibility concerns. However, we still want to adopt the new formatter as a standard across all packages.

Allowing a formatter.version parameter would let developers opt into the new formatting rules while maintaining backward compatibility.