Supertypes are a honking great idea. We should use more of them.
This massively cleans up the node types, without polluting the AST with
`expression` nodes.
Before, the `condition` field of an if statement supposedly could
contain things like parentheses and commas, due to bleeding from
referenced anonymous nodes. Making the node named makes this issue go
away.
We make _referenceable_operator a named node. This prevents it from
bleeding through to the _expression definition. It likely also makes the
output easier to deal with, as bare operators used as arguments now have
a named node wrapping them in the AST.
Also removes a duplicated inclusion of _comparison_operator that served
no purpose.
This caused any field containing an _expression to appear as if it could
countain any number of such nodes. It also threw away the information
that there was a `?` marker there.
To fix it, we simply move the definition into its own named node.
The astute reader will note that we seem to _lose_ some node types in
the process. Apparently, these were unreachable in the grammar, and the
newer version of tree-sitter removes such "dead code".
