[Indic] Allow a ZWNJ after SM's

Malayalam failures go way down.  Other scripts benefitted slightly too.
Sinhala had one or two test regressions, but...

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

@@ -65,7 +65,7 @@ reph = (Ra H | Repha);		# possible reph
 cn = c.n?;
 forced_rakar = ZWJ H ZWJ Ra;
 matra_group = z*.M.N?.(H | forced_rakar)?;
-syllable_tail = SM? (Coeng (cn|V))? (VD VD?)?;
+syllable_tail = (SM.ZWNJ?)? (Coeng (cn|V))? (VD VD?)?;
 place_holder = NBSP | DOTTEDCIRCLE;
 halant_group = (z?.h.ZWJ?);
 final_halant_group = halant_group | h.ZWNJ;