[Indic] Relax grammar

Now that we insert dotted-circle, tests break more easily when our indic
machine breaks.

In particular, a few Devanagari tests were having sequences like
"C,H,ZWJ,N", and because of the ZWJ the Nukta does NOT get reordered to
before the Halant as the grammar used to expect...  Fixup.

Another case is as simple as "C,ZWJ,SM".

Fixes 10 out of 79 failures:

DEVANAGARI: 707325 out of 707394 tests passed. 69 failed (0.00975411%)

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

@@ -62,12 +62,12 @@ z = ZWJ|ZWNJ;			# is_joiner
 h = H | Coeng;			# is_halant_or_coeng
 reph = (Ra H | Repha);		# possible reph
 
-cn = c.n?;
+cn = c.ZWJ?.n?;
 forced_rakar = ZWJ H ZWJ Ra;
 matra_group = z{0,3}.M.N?.(H | forced_rakar)?;
 syllable_tail =  (Coeng (cn|V))? (SM.ZWNJ?)? (VD VD?)?;
 place_holder = NBSP | DOTTEDCIRCLE;
-halant_group = (z?.h.ZWJ?);
+halant_group = (z?.h.(ZWJ.N?)?);
 final_halant_group = halant_group | h.ZWNJ;
 halant_or_matra_group = (final_halant_group | matra_group{0,4});