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/Runtime/Intl/RelativeTimeFormat.cpp

@@ -148,7 +148,7 @@ ThrowCompletionOr<Vector<PatternPartitionWithUnit>> partition_relative_time_patt
             VERIFY(patterns.size() == 1);
 
             // i. Let result be patterns.[[<valueString>]].
-            auto result = patterns[0].pattern.to_string();
+            auto result = patterns[0].pattern.to_deprecated_string();
 
             // ii. Return a List containing the Record { [[Type]]: "literal", [[Value]]: result }.
             return Vector<PatternPartitionWithUnit> { { "literal"sv, move(result) } };