[Indic] Treat Khmer Register Shifters more like Nuktas

Except that there may be a ZWNJ before a Register Shifter.

src/hb-ot-shape-complex-indic-machine.rl

@@ -56,7 +56,7 @@ RS   = 14;
 Coeng = 15;
 
 c = C | Ra;
-n = N N?;
+n = (N.N? | ZWNJ?.RS);
 z = ZWJ|ZWNJ;
 h = H | Coeng;
 matra_group = (M | RS) N? H?;

test/shaping/texts/in-tree/shaper-indic/south-east-asian/script-khmer/misc/misc.txt

@@ -12,3 +12,5 @@
 ង្ឈឹ
 ង្គ្រ
 ង្រ្គ
+ម៉្លេះ
+ម‌៉្លេះ