LibUnicode: Parse and generate format pattern skeletons

Pattern skeletons are more or less the "key" of format patterns. Every
format pattern is assigned a skeleton. Interval patterns (which are not
yet parsed) are also assigned a skeleton - this is used to match them to
an "owning" format pattern. So we will use the skeleton generated here
to match format patterns at runtime with their available interval
patterns.

An alternative approach would be to append interval patterns directly to
their owning format pattern, but this has some draw backs:

    1. Skeletons aren't totally unique. A skeleton may appear in both
       the "dateFormats" and "availableFormats" objects, in which case
       the same interval formats would be generated more than once.

    2. Otherwise unique format patterns may only differ by the interval
       patterns assigned to them. This would cause the UniqueStorage for
       the format patterns to increase in size, impacting both compile
       times and libunicode.so size.

Userland/Libraries/LibUnicode/DateTimeFormat.h

@@ -100,6 +100,7 @@ struct CalendarPattern {
         callback(time_zone_name, other.time_zone_name, Field::TimeZoneName);
     }
 
+    String skeleton {};
     String pattern {};
     Optional<String> pattern12 {};