LibUnicode: Generate General Category unions and aliases

This downloads the PropertyValueAliases.txt UCD file, which contains a
set of General Category aliases.

This changes the General Category enumeration to now be generated as a
bitmask. This is to easily allow General Category unions. For example,
the LC (Cased_Letter) category is the union of the Ll, Lu, and Lt
categories.

Userland/Libraries/LibUnicode/Forward.h

@@ -11,7 +11,7 @@
 namespace Unicode {
 
 enum class Condition;
-enum class GeneralCategory;
+enum class GeneralCategory : u64;
 enum class Locale;
 enum class Property : u64;
 enum class WordBreakProperty;