[api_summary] Track pubspec.yaml environment constraints

[kevmoo]

Currently, api_summary does not check the environment section of pubspec.yaml. Bumping any environment constraint (sdk, flutter, or other engines/platforms) restricts downstream consumers running on older SDK/engine releases, which constitutes a breaking change for the package version.

Example

Changing:

environment:
  sdk: ">=3.0.0 <4.0.0"
  flutter: ">=3.10.0"

to:

environment:
  sdk: ">=3.1.0 <4.0.0"
  flutter: ">=3.16.0"

currently produces identical api_summary output.