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/HTMLElement.cpp

@@ -157,7 +157,7 @@ DeprecatedString HTMLElement::inner_text()
     };
     recurse(*layout_node());
 
-    return builder.to_string();
+    return builder.to_deprecated_string();
 }
 
 // // https://drafts.csswg.org/cssom-view/#dom-htmlelement-offsettop