First, we were abusing OT_VD instead of OT_A.  Fix that
but moving OT_A in the grammar where it belongs (which
is different from what the spec says).

Also, only allow medial consonants after all other
consonants.  This doesn't affect any current character.

Finally, fix Halant attachment in presence of medial
consonants.  Again, this currently doesn't affect any
sequence.

I lied.  There's Gurmukhi U+0A75 which is Consonant_Medial.
Uniscribe allows one of those in each of these positions:
before matras, after matras and before syllable modifiers,
and after syllable modifiers!  We currently just allow
unlimited numbers of it, before matras.