Support extension types in checks codegen

[natebosch]

The element model normalizes extension types to their representation
type, so we need to use the AST to read field names and types for
extension types.

Initial vibe-coded approach - will need review/refactor before landing.

[github-actions]

PR Health

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:checks	0.3.2-wip	WIP (no publish necessary)
package:checks_codegen	0.1.0	ready to publish	checks_codegen-v0.1.0
package:fake_async	1.3.3	already published at pub.dev
package:matcher	0.12.20	already published at pub.dev
package:test	1.31.2-wip	WIP (no publish necessary)
package:test_api	0.7.13-wip	WIP (no publish necessary)
package:test_core	0.6.19-wip	WIP (no publish necessary)
package:test_descriptor	2.0.2	already published at pub.dev
package:test_process	2.1.1	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.