[khmr/km] Please include coeng forms of independent vowels

[KFS-Typography]

Comment on http://r12a.github.io/scripts/khmr/

Independent vowels also have coeng form

Although very rare and highly discouraged, they are still acceptable spelling and part of the Chuon Nath Khmer dictionary that further explains the root word origins from its original Sanskrit orthography rendered in Khmer. Sanskrit and Pali dictionaries would contain even more use cases for coeng form of independent vowels.