Add `platform` declaration to pubspec.yaml

[sidrao2006]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Platform support declaration for pub.dev

:arrow_heading_down: What is the current behavior?

pub.dev incorrectly identifies platform support for flutter_cache_manager

:new: What is the new behavior (if this is a feature change)?

Supported platforms are declared in pubspec.yaml

:boom: Does this PR introduce a breaking change?

No

:bug: Recommendations for testing

N/A

:memo: Links to relevant issues/docs

https://dart.dev/tools/pub/pubspec#platforms

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines (code style guide)
• [ ] Relevant documentation was updated
• [ ] Rebased onto current develop