ar: introduce group for diacritics

This group is used as a padding between consonant letter and its other diacritics
which may be in arbitrary order

dictsource/ar_rules

 // Arabic Moon characters
 .L11  ق ف غ ع ه خ ح ج ي ب م ك  و  إ أ ء
 
+// Arabic diacritics
+.L20  ۡ ـ  ً  ٌ  ٍ  َ  ُ  ِ  ّ  ْ
+
 .group آ
          آ           ?aa