[indic] Disallow vowel mark combinations that spoof other vowel marks

[PeterCon]

Some people try to use vowel mark combinations like <consonant, U+093E, U+0947> to give the appearance of a different vowel, in this example, <consonant, U+094B>. Vowel mark combinations that spoof other vowel marks.

This is one more variation on the issue called out in Unicode 10, Chapter 12, Table 12-1: text should be encoded using atomic characters, not sequences that analyze the character in terms of visual combinations of other characters. Issue 556 is another variant of the same principle (and Unicode 11 will include additional text in Chapter 12 addressing that).