Create vidyut-akshara crate

[akprasad]

Several of our crates have some version of a struct Set and other utilities for working with Sanskrit sounds. These should probably be their own crate. We can it call it vidyut-akshara or whatever name feels right, and we can include:

• Sound
• Map
• is_ac, is_hal
• const AC, const HAL

etc.

I'm thinking about this in relation to #180 from @vbasky, since many of our uses of OnceLock would go away if we could use the const version of struct Set defined in vidyut-prakriya.