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/CSS/Resolution.cpp

@@ -21,7 +21,7 @@ Resolution::Resolution(float value, Type type)
 {
 }
 
-DeprecatedString Resolution::to_string() const
+DeprecatedString Resolution::to_deprecated_string() const
 {
     return DeprecatedString::formatted("{}{}", m_value, unit_name());
 }