kmk.consts: rename UnicodeModes => UnicodeMode

This brings this naming into consistency with both fellow consts in the
same file (ex. LeaderMode is singular) as well as the variables in which
the consts are usually used (usually a `Firmware.unicode_mode` attribute
in a keymap).

kmk/consts.py

@@ -142,7 +142,7 @@ class DiodeOrientation:
     ROWS = 1
 
 
-class UnicodeModes:
+class UnicodeMode:
     NOOP = 0
     LINUX = IBUS = 1
     MACOS = OSX = RALT = 2