unicode: add CategoryAliases, Cn, LC

CategoryAliases is for regexp to use, for things like \p{Letter} as an alias for \p{L}.
Cn and LC are special-case categories that were never implemented
but should have been.

These changes were generated by the updated generator in CL 641395.

Fixes #70780.

Change-Id: Ibba20ff76191c8ae9631ac5ba19965790fe0cc81
Reviewed-on: https://go-review.googlesource.com/c/go/+/641376


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>