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/LibSQL/AST/Parser.h

@@ -26,7 +26,7 @@ class Parser {
         DeprecatedString message;
         SourcePosition position;
 
-        DeprecatedString to_string() const
+        DeprecatedString to_deprecated_string() const
         {
             return DeprecatedString::formatted("{} (line: {}, column: {})", message, position.line, position.column);
         }