[css-fonts] Add generic font family, Khmer Mul

[r12a]

https://www.w3.org/TR/css-fonts-5/#generic-font-families
https://www.w3.org/TR/css-fonts-4/#generic-font-families

Per #8128 (comment) i would like to propose a new generic font label for Khmer âksâr mul (pronounced /ʔɑːksɑː muːl/) font style, which is commonly used for headings and other distinctive text in Khmer. For more information, see https://r12a.github.io/scripts/khmr/km.html#writingstyles

'Mul' is spelled in a variety of ways, including 'mul', 'mool', 'moul', and 'muul'. I'm not sure whether the name for the font family label is sufficient as just 'mul', or whether it should be 'khmer-mul', or perhaps 'askar-mul'.

Khmer has some other font styles, too, but this seems to be the one you'd most likely want to be able to control fallback behaviour for.