[indic] Rename avagraha cluster to symbol cluster

In anticipation of adding more characters to that class of clusters.
[indic] Always start new syllable for Avagraha

In fact, the previous grammar was ambigious.  No functional
change.
[indic] Simplify grammar

No functional change.
[indic] Support U+A8F2..A8F7,1CE9..1CEC,1CEE..1CF1
[indic] Support U+1CED