SwiftExtract: include URL in the Foundation known-module overlay

The Foundation/FoundationEssentials known-module source files declared
Data, Date, and UUID, but URL was missing — so any user code using URL
(e.g. `func getHost(url: URL)`) failed to import with an unresolved-type
warning, dropping the function silently. swift-python relies on URL for
its bridging tests; add it alongside the other Foundation built-ins,
declaring just the failable `init(string:)` and `absoluteString`
property the language-neutral analyzer needs to resolve uses.

Author: ktoso

Sources/SwiftExtract/SwiftTypes/SwiftKnownModules.swift

@@ -130,6 +130,11 @@ private let foundationEssentialsSourceFile: SourceFileSyntax = """
   }
 
   public struct UUID {}
+
+  public struct URL {
+    public init?(string: String)
+    public var absoluteString: String { get }
+  }
   """
 
 private var foundationSourceFile: SourceFileSyntax {