feat: v0.1.0 - simplify form APIs, support Dart 3.13 + analyzer 13.x

[jeroen-meijer]

Note

I no longer have push access to this repo. The branch now lives at jeroen-meijer/shape@feat/v0.1.0.

Description

This PR reworks the shape and shape_generator APIs for v0.1.0, cutting form boilerplate while keeping generated behavior explicit and tested.

API changes

• Remove FormBody and FormErrors generic type parameters from the public API
• Add SimpleFormField<T, E> and @FieldRequired() for required validation on optional params
• Support redirecting factories and automatic GenericFormField wrapping for plain params in the generator
• Keep explicit custom field wrappers in generated form bodies
• Drop Equatable from the generated/public API path; generate operator == and hashCode instead
• Better constructor and field validation diagnostics in shape_generator
• Update examples, READMEs, root migration docs, fixtures, goldens, and test harnesses
• Add and update tests across shape, shape_generator, and example package
• Add changelog entries for API, generator, and internal changes

Dart 3.13 / analyzer upgrade

• SDK constraint bumped to >=3.13.0 across all packages
• shape_generator: analyzer bumped to >=13.3.0 <15.0.0, build to ^4.0.8
• Migrate NamedExpression to NamedArgument API (name.label.name to name.lexeme, expression to argumentExpression)
• Skip primary constructors during AST resolution (no ConstructorDeclaration node for those)
• Remove Flutter 3.44 compat pins on test, build_runner, build