Protowhat

- Support Python version 3.9
- Update dependencies to up-to-date versions

- Expose `prepare_validation` from `protowhat.checks`

- Refactor SCT syntax to be an execution graph and enable derived features
- Split SCT syntax and SCT context code
- Refactor exceptions to share an interface to simplify logic
- Refactor feedback to be centralized and built from formalized components
- Add support for embedding xwhats
- Add `prepare_validation` helper for checking bash history
- Add autodebug (`_debug` will be invoked automatically for instructor errors)

- Fix final feedback message after append_messsage(s) from having duplicate information

- Add (default off) append_message functionality to check_node, check_edge and has_eqaul_ast

- Add include_head option to Selector