[indic] Allow up to two syllable modifiers

Bug 70509 - Candrabindu+Visarga doesn't work in Devanagari
https://bugs.freedesktop.org/show_bug.cgi?id=70509

We categorize both bindus and visarga as syllable-modifiers.
OT spec doesn't actually say what characters go in the syllable
modifier category, and allows one.  We just allow up to two now.

Test case: U+0930,U+0941,U+0901,U+0903

Uniscribe currently doesn't support that and produces a
dotted circle.

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

@@ -67,7 +67,7 @@ reph = (Ra H | Repha);		# possible reph
 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))? (Avag.N?)? (SM.ZWNJ?)? (VD VD?)?;
+syllable_tail =  (Coeng (cn|V))? (Avag.N?)? (SM.SM?.ZWNJ?)? (VD.VD?)?;
 place_holder = NBSP | DOTTEDCIRCLE;
 halant_group = (z?.h.(ZWJ.N?)?);
 final_halant_group = halant_group | h.ZWNJ;

test/shaping/texts/in-tree/shaper-indic/indic/script-devanagari/misc/misc.txt

@@ -31,3 +31,4 @@
 र्आ्र्
 क‌ि
 ऽं
+रुँः