Add Indic_Conjunct_Break property.

[makotokato]

From #6367, @eggrobin suggests to use Indic_Conjunct_Break (InCB) property for Grapheme Cluster Break.

Also, InCB.toml is incomplete yet like the following, since it is added by ICU76 as a draft API.

values = [
  {discr = 0, long = "None", short = "None"},
]

It means that names (short / long / parse) are empty for this implementation.

[makotokato]

After fixing https://unicode-org.atlassian.net/browse/ICU-23092 / unicode-org/icu#3457, icuexport will export valid value for Indic_Conjunct_Break.

[Manishearth]

Code lgtm

Given that this doesn't have names and is a draft property, should we make this doc(hidden) for now until it has names? This is only needed internally for segmenter, so we don't have to commit to the property enum just yet if we don't want to.

cc @sffc @robertbastian @hsivonen

[robertbastian]

We can remove names from the API. I'm happy to expose this as a stable (nameless) property.

[makotokato]

Does it mean that we can remove parse, short name and long name?

[robertbastian]

I think that would be the best option for now.

[Manishearth]

Also cc @sffc for doc(hidden)