Track attr definition sites for duplicate attribute errors

[robrix]

When I define an attribute that turns out to be a duplicate, it can be a substantial amount of effort to track out where the original assignment was, especially if other kinds of nodes use the same attribute names. It'd be nice if tree-sitter-graph were to keep track of where (in the rules file's source) attributes were created so they could be reported for debugging.

Hypothetically we could compute this statically given the grammar (or at least an overapproximation to it), but I think we'd be better off doing it dynamically at least for the time being. If it's too slow to do all the time, maybe we could turn it on with a flag.