Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibCodeComprehension/Cpp/CppComprehensionEngine.h

@@ -40,7 +40,7 @@ private:
         static SymbolName create(StringView, Vector<StringView>&&);
         static SymbolName create(StringView);
         DeprecatedString scope_as_string() const;
-        DeprecatedString to_string() const;
+        DeprecatedString to_deprecated_string() const;
 
         bool operator==(SymbolName const&) const = default;
     };