add API for Indic_Positional_Category & Indic_Syllabic_Category

See http://www.unicode.org/reports/tr44/#IndicPositionalCategory.txt

For completeness, we probably want to provide API for these recently added enumerated properties. However, we might want to wait until they are not "provisional" properties any more; or at least, keep the API @draft while they are.

Implementation-wise, see if it's better to a) widen the uprops.icu properties vectors (adding 4+5 bits) or b) have a simple/special data structure like for the Joining_Group.