Qualified Name Lookup for swift-syntax

I propose reimplementing the compiler’s qualified name lookup in the Swift-only "swift-syntax" package. I will add a new API to the SwiftLexicalLookup module that accepts a declaration group, such as a type, and returns its members —commonly properties and functions. Lifting this API into the higher-level SwiftSyntax package exposes essential semantic information, enabling build tools that perform more intricate code transformations and offer more precise diagnostics. The library will find a declaration group's members through the package's SyntaxVisitor API and optionally cache results in a new SymbolTable type. I will deliver the updated SwiftLexicalLookup library with helpful docstrings and comments, extensive unit testing, and unqualified-lookup integration, along with two dedicated documentation articles.