Add support to copy objc selector

[iMostfa]

Resolves: swiftlang/sourcekit-lsp#2145
along with: swiftlang/sourcekit-lsp#2399

example.mov

Add Copy Objective-C Selector Refactoring

This PR adds a new refactoring action that allows users to copy the Objective-C selector from Swift methods marked with @objc or @objcMembers

• New SourceKit request: source.request.objc.selector - retrieves the Objective-C selector for a method at a given position
• Refactoring support: Integrated into the semantic refactoring system to provide "Copy Objective-C Selector" as a refactoring action
• Test coverage: Added comprehensive tests covering simple methods, methods with parameters, and methods with custom selectors

SourceKit API (SwiftSourceDocInfo.cpp:2888-2974):

• New getObjCSelector method that resolves cursor position to an AbstractFunctionDecl
• Validates the declaration has @objc annotation
• Extracts and returns the selector string using getObjCSelector().getString()

Request Handling (Requests.cpp:1767-1813):

• Added handleRequestGetObjCSelector to process selector extraction requests
• Returns selector as text via key.text in the response