support ? as punctuation in NodeName

[vidhanio]

this would be useful for marking "optional" attributes. is this possible?

[vldm]

I think this is a part of #8
Currently puncts is not allowed in attribute names. I think a good starting point would be to list all possible symbols in attribute names, most of them can already be implemented, but some conflicts with syn::Expr parsing.

As a workaround what do you think about moving "optional" attribute syntax to the "value" part?
foo={?x>2} - this is already doable through transform block.