Implement augmentations feature

[itsjustkevin]

This is an umbrella issue covering augmentations.

Generated issue links:

• [augmentations] Write the spec #56151
• [augmentations] Language tests #56152
• [augmentations] Co19 tests - add tests #56153
• [augmentations] Implement feature in the CFE #56154
• [augmentations] Analyzer implementation #56155
• [augmentations] Tests validating as passing (Tracking via Google Sheet) #56156
• [augmentations] Syntax highlighters implementation #56157
• [augmentations] DDC implementation #56158
• [augmentations] Dart2js implementation #56159
• [augmentations] VM implementation #56160
• [augmentations] Build system support #56161
• [augmentations] Web debugging support #56162
• [augmentations] Core library updates #56163
• [augmentations] Analysis server / IDE #56164
• [augmentations] dart_style support #56165
• [augmentations] dartdoc implementation #56166
• [augmentations] Cider support #56167
• [augmentations] Kythe implementation #56168
• [augmentations] Angular compiler implementation #56169
• [augmentations] Intellij implementation #56170
• [augmentations] Language feature docs #56171
• [augmentations] VM service protocol support #56172
• [augmentations] VM debugging support #56173
• [augmentations] linter support #56174

[dart-github-bot]

Summary: This issue is a placeholder for a larger effort to implement augmentations, a new feature that allows extending existing classes without modifying their source code. It is broken down into several sub-issues, each addressing a specific aspect of the feature.