[Breaking] Replace usages of dart:math geometry types with custom types

[parlough]

This is a breaking change and shouldn't be included until the next major release.

These types were built for dart:html, contain unrelated functionality, and require this library to always add an int generic. To aid the deprecation of these types in dart:math, migrate this package to custom types. These types, particularly the addition of a new Size type can more semantically match their purpose in the package, and be a bit simpler to use, no longer having a type parameter.

Contributes to dart-lang/sdk#54852

[devoncarew]

I don't know that I have enough context on the 'why' to evaluate this PR. Perhaps we should resolve dart-lang/sdk#54852 first? cc @kevmoo for thoughts

[parlough]

I don't know that I have enough context on the 'why' to evaluate this PR. Perhaps we should resolve dart-lang/sdk#54852 first?

That issue is the why for this. This package seems to be the only dependency of Flutter that uses these types, being a blocking stop in deprecating the types.

Happy to continue the discussion in that issue first though :)