[Patterns] Analysis server / IDE support work

[itsjustkevin]

This is the issue tracking the work that needs to be done in the analysis server to support the patterns feature. If you decide to work on a feature, either create an issue for the feature and link to it from here, or put your name beside the feature so that we don't duplicate effort.

• [ ] Call Hierarchy
• [ ] Closing Labels
• [ ] Code Completion
• [ ] Code Folding
• [ ] documentSymbol
• [ ] Flutter Outline
• [ ] Hovers
• [ ] Implemented Markers
• [ ] Navigation
• [ ] Occurrences
• [ ] Organize Imports
• [ ] Outline
• [ ] Overrides Markers
• [ ] #49960
• [ ] Refactorings - legacy
• [ ] Refactorings - self describing
• [ ] Search - Find References
• [ ] Search - Member Declarations
• [ ] Search - Member References
• [ ] Search - Top-level Declarations
• [ ] selectionRange
• [ ] Semantic Highlights
• [ ] signatureHelp
• [ ] Sort Members