Update dependency code_builder to v4

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
code_builder	dependencies	major	^3.2.1 -> ^4.0.0

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Release Notes

dart-lang/code_builder

v4.4.0

Compare Source

• Mention how the allocator argument relates to imports in the DartEmitter
  constructor doc.
• Add support for emitting typedefs.
• Add support for emitting leading line comments for libraries.
• Add support for emitting ignore_for_file analyzer directive comments.

v4.3.0

Compare Source

• Add support for adding more implementation in enum classes.
• Only emit late keyword when using null safety syntax.
• Use implicit const when assigning to a declareConst variable.
• Deprecate assignVar, assignConst, and assignFinal.
• Add trailing commas to any parameter list, argument list, or collection
  literal which has more than one element.

v4.2.0

• Add an ignore for a lint from the package:lints recommended set. The lint,
  no_leading_underscores_for_library_prefixes is most useful for hand edited
  code where the appearance of a private name that is already not visible
  outside the library is confusing.
• Fix the docs for Expression.assign, ifNullThen, and assignNullAware
  which had the argument and receiver flipped.
• Add declareConst, declareFinal, and declareVar to replace
  Expression.assignConst, assignFinal, and assignVar. Add support for late
  variables with the declare* utilities.
• Add ParameterBuilder.toSuper so support super formal parameters language
  feature.

v4.1.0

• Add Expression.spread for the spread operator ....
• Add support 'late' field modifier.
• Add support for Expression.nullChecked to add a null assertion operator.
• Add support for creating mixins.
• Add Expression.nullSafeSpread for the null aware spread operator ...?.
• A Library can now be annotated.

v4.0.0

Compare Source

• Migrate to null safety.
• Changed the DartEmitter constructor to use named optional parameters.
• Add ParenthesizedExpression and
  ExpressionVisitor.visitParenthesizedExpression.

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