[JuliaLowering] Clean up `nothing` representation (#61549)
Fix https://github.com/JuliaLang/JuliaLowering.jl/issues/173 "the right
way" by hopefully eliminating a footgun: SyntaxTree can now store the
value `nothing` in an attribute, though in practice JL won't do this
outside of K"Value", so things like `get(st, :name_val, nothing)` are
still fine to do.
`nothing::K"Value"` is a valid representation of `nothing` throughout
lowering, but I've also added `K"nothing"` (analogous to flisp's
`(null)` form) since lowering matches on it, and when we're peeking into
`.value` or `.name_val` for a specific thing, we should usually be using
a special form instead. This form is not valid before desugaring (it's
converted from `nothing::K"Value"` in `est_to_dst`), so it shouldn't
cause confusion in "new" macro expansion.
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Co-authored-by: Cody Tapscott <topolarity@tapscott.me>
