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/LibWeb/HTML/HTMLFormElement.cpp

@@ -158,7 +158,7 @@ DeprecatedString HTMLFormElement::action() const
 
     // Return the current URL if the action attribute is null or an empty string
     if (value.is_null() || value.is_empty()) {
-        return document().url().to_string();
+        return document().url().to_deprecated_string();
     }
 
     return value;