Umbrella issue to track all of the Objective-C to Swift migration work within Flutter and 1p Flutter packages

• [packages] Add support for swift-format #41129
• Build Swift files in the engine to support Swift-only APIs on iOS and macOS #144791
• Deprecate iOS Objective-C create template flutter create --ios-language objc for new apps #148586
• Deprecate iOS Objective-C create template flutter create --ios-language objc -t plugin for new plugins #169683
• Support creation of add-to-app modules in Swift #23955
• [plugins] ☂️ Swift migration for 1P plugins #119015
• [ios] [integration_test] avoid using preprocessor macros to make it easier to use from Swift #71873
• Add swift tests for the flutter framework #137368
• [pigeon] Consider removing Obj-C generator #158288
• Format and lint Swift files with swift-format #172799
• Migrate Flutter engine code from Objective-C to Swift #175878

Note: developers will continue to be able to mix Swift and Objective-C in projects and packages as long as Apple (Xcode, Swift Package Manager, etc) allows it. However Flutter will no longer generate Objective-C code, it will be up to developers to handle Objective-C integration themselves, as with any native darwin project.