Customize the PEGTL parse_tree implementation

[wravery]

Pending taocpp/PEGTL#309 (or perhaps taocpp/PEGTL#313), I need to suppress the try/catch/rethrow handling in match_control_unwind to unblock reasonably deep nested selection sets on Windows. This was originally reported as #222, which was mitigated by adding a depth limit parameter, but it still needs to be set to a very low number to avoid a stack overflow on Windows. Since ast_node does not have any unwind handling, it is safe to let the exception unwind directly from where it is thrown to the point where we're attempting to parse a GraphQL document. If/when that PR is merged, I can revert to the original version in PEGTL.

This also simplifies a lot of the ast_node implementation by inheriting from PEGTL's parse_tree::basic_node CRTP type. This will help when migrating to the 4.x/main branch of PEGTL because some of the input iterator types have been replaced with frobnicator types, indicating that they should be considered a low-level internal feature and not something that should be referenced outside of it.

If for some reason taocpp/PEGTL#313 is merged without taocpp/PEGTL#309, then I will need to make some more changes before I can revert the custom parse tree. By inheriting from parse_tree::basic_node, I'm exposing an unwind implementation on ast_node. I will likely need to compose the parse_tree::basic_node as a private member instead of inheriting it to be able to suppress the call to unwind with taocpp/PEGTL#313 by itself.