Implement the enhanced-parts feature

[eernstg]

This issue is used to track the implementation of the 'enhanced-parts' feature.

Feature specification: https://github.com/dart-lang/language/blob/main/working/augmentation-libraries/parts_with_imports.md.

Experiment flag: enhanced-parts.

Subtasks:

• https://github.com/dart-lang/sdk/issues/56190
• https://github.com/dart-lang/sdk/issues/56191
• https://github.com/dart-lang/sdk/issues/56192
• https://github.com/dart-lang/sdk/issues/56193
• https://github.com/dart-lang/sdk/issues/56194
• https://github.com/dart-lang/sdk/issues/56195
• https://github.com/dart-lang/sdk/issues/56196
• https://github.com/dart-lang/sdk/issues/56197
• https://github.com/dart-lang/sdk/issues/56198
• https://github.com/dart-lang/sdk/issues/56199
• https://github.com/dart-lang/sdk/issues/56200
• https://github.com/dart-lang/sdk/issues/56201
• https://github.com/dart-lang/sdk/issues/56202
• https://github.com/dart-lang/sdk/issues/56203
• https://github.com/dart-lang/sdk/issues/56204
• https://github.com/dart-lang/sdk/issues/56205
• https://github.com/dart-lang/sdk/issues/56206
• https://github.com/dart-lang/sdk/issues/56207
• https://github.com/dart-lang/sdk/issues/56208
• https://github.com/dart-lang/sdk/issues/56209
• https://github.com/dart-lang/sdk/issues/56210
• https://github.com/dart-lang/sdk/issues/56211
• https://github.com/dart-lang/sdk/issues/56212
• https://github.com/dart-lang/linter/issues/5024

[dart-github-bot]

Summary: This issue is a placeholder for a larger feature implementation, "enhanced-parts," which is broken down into multiple sub-issues. The sub-issues cover various aspects of the feature, including its design, implementation, and testing.