Make Indic_Conjunct_Break property public / stable

[makotokato]

Follow up #6379. After ICU4C's InCB is public, we also make it public by removing #[doc(hidden)].

Also, if unicode-org/icu#3457 is fixed in ICU4C stable release, we should add an parser implementation for UTR#35 etc.

[Manishearth]

Status: we implement the property getter as a hidden unstable API. We plan to stabilize once it is stable in ICU4C.