chore(deps): update dependency source_gen to v4

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
source_gen (source)	dependencies	major	^3.0.0 -> ^4.0.0

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Release Notes

dart-lang/source_gen (source_gen)

v4.0.2

Compare Source

• Bug fix: fix possible null pointer exception in TypeChecker.typeNamed on
  invalid code.

v4.0.1

Compare Source

• Require analyzer: '>=8.1.1 <9.0.0'.
• Allow build: '>=3.0.0 <5.0.0'.

v4.0.0

Compare Source

• Breaking Change: remove TypeChecker.fromRuntime, use
  TypeChecker.typeNamed instead. This removes all use of dart:mirror, so
  builders using source_gen can be AOT compiled for better performance.
• Keep // GENERATED FILE comments on the first line.

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