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.

Tests/LibCpp/test-cpp-preprocessor.cpp

@@ -48,7 +48,7 @@ TEST_CASE(test_regression)
 
         EXPECT_EQ(tokens.size(), target_lines.size());
         for (size_t i = 0; i < tokens.size(); ++i) {
-            EXPECT_EQ(tokens[i].to_string(), target_lines[i]);
+            EXPECT_EQ(tokens[i].to_deprecated_string(), target_lines[i]);
         }
     }
 }