Implement CompilerDesugaringKind enum
This is the first step outlined in #35946. I think that the variants of `CompilerDesugaringKind` should be changed, I didn't know what the official names for `...` and `<-` are.
I'm not to sure how tests for the compiler work, but I would imagine that tests should be added such that
`Symbol::intern(s) == CompilerDesugaringKind::from(s).as_symbol()` for valid `s`.