NLS: missing singleton compose sequences that take advantage of `ch`/`Ch`/`CH`/`c_h`/`C_h`/`C_H` keysyms

The Breton keyboard still uses PUA characters to expand single keystrokes to "ch"/"Ch"/"CH"/"c’h"/"C’h"/"C’H" in a user-defined XCompose file. There are currently named keysyms for ch/Ch/CH/c_h/C_h/C_H which are as of yet unused. I believe there ought to be some singleton key sequences in the standard Compose file which expand the named keysyms to the full strings so that the Breton keyboard can actually use the named keysyms and work out-of-the-box.