Leaf kinds we don't expect

[mlechu]

julia> jlower("&")
ERROR: LoweringError:
&
╙ ── invalid syntax

julia> jlower("function (&)(::Int); 1; end")
ERROR: LoweringError:
function (&)(::Int); 1; end
#         ╙ ── Invalid function name

&, unless it's being called, is K"&" instead of K"Identifier". | and : behave the same. Are there others? How do I find out?

I think I'll fix this by converting any unknown leaf kind with a name_val to a K"Identifier" in EST so they're more consistent with Expr and easier to handle. Opening an issue to remember, and to consider changing the text->RawGreenNode behaviour instead since this could be called a parser bug.