Document new Unicode class

doc/Type/Unicode.pod6

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+=begin pod :kind("Type") :subkind("class") :category("domain-specific")
+
+=TITLE class Unicode
+
+=SUBTITLE Unicode related information
+
+    class Unicode { }
+
+Built-in class for providing Unicode related information.  Although it
+can be instantiated, these methods currently mostly make sense when called
+as class methods.  In which case they represent the information of the
+supported version of Unicode in the current runtime.
+
+Available as of release 2023.02 of the Rakudo compiler.  Available as an
+L<installable module|https://raku.land/zef:lizmat/Unicode> for earlier
+versions of the Rakudo compiler.
+
+=head1 Methods
+
+=head2 method version
+
+    method version(Unicode:)
+
+Returns a L«C<Version>|/type/Version» object representing the Unicode
+version.
+
+    say Unicode.version; # OUTPUT: «v15.0␤»
+
+=head2 method NFG
+
+    method NFG(Unicode:)
+
+Returns a L«C<Bool>|/type/Bool» indicating whether complete
+L«C<Normalization Form Grapheme>|/language/glossarye#NFG» support is
+available.
+
+    # on MoarVM
+    say Unicode.NFG; # OUTPUT: «True␤»
+
+    # on JVM
+    say Unicode.NFG; # OUTPUT: «False␤»
+
+=end pod