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/LibJS/Contrib/Test262/$262Object.cpp

@@ -97,7 +97,7 @@ JS_DEFINE_NATIVE_FUNCTION($262Object::eval_script)
         auto& error = script_or_error.error()[0];
 
         // b. Return Completion { [[Type]]: throw, [[Value]]: error, [[Target]]: empty }.
-        return vm.throw_completion<SyntaxError>(error.to_string());
+        return vm.throw_completion<SyntaxError>(error.to_deprecated_string());
     }
 
     // 5. Let status be ScriptEvaluation(s).