LibJS: Change Intl's PatternPartition record to hold a String value

It was previously holding a StringView, which was either a view into a
LibUnicode-generated string or a string passed from the user.

Intl.NumberFormat will need this record to hold internally-created
strings, so a StringView will not suffice (the way the steps are laid
out, that view will ultimately end up dangling).

This shouldn't be too wasteful since the StringView it was holding was
converted to a String eventually anyways.

Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.h

@@ -31,7 +31,7 @@ struct LocaleResult {
 
 struct PatternPartition {
     StringView type;
-    StringView value;
+    String value;
 };
 
 Optional<Unicode::LocaleID> is_structurally_valid_language_tag(StringView locale);