Sokoenginepy

- maintenance release, switch from TravisCI to GitHub Actions
- fully automated deployment of C++ docs based on `development` branch

- added: `BoardGraph.cell_orientation()`
- build: optimized Python C++ extension binary size

- added: `BoardGraph.wall_neighbor_directions()`
- added: `Puzzle.has_sokoban_plus`
- renamed: `Collection.save()` -> `Collection.dump()`
- changed: `Collection.dump()` can now save to IO streams (previously it could only save
to file path)
- changed: `Collection.load()` can now load from IO streams (previously it could only load
from file path)
- added: `Collection.loads()` and `Collection.dumps()`
- changed: `Collection.notes`, `Puzzle.notes` and `Snapshot.notes` are now ordinary
strings (instead of list of strings)
- build: `pip` and `python -m build` now use `cmake` instead of `distutils`
- build: `vcpkg` is used for all C++ dependencies

- renamed `DEFAULT_PIECE_ID` -> `DEFAULT_ID`
- renamed index functions
- `X()` -> `index_x()`
- `Y()` -> `index_y()`
- `ROW()` -> `index_row()`
- `COLUMN()` -> `index_column()`
- replaced use of `None` in method arguments and returned values with explicit constants
- ie. `BoardGraph.neighbor()` now always returns `int` and instead of `None` it
returns `Config.NO_POS`.
- this makes APIs easier to use and pybind11 bridge more efficient (because pybind11
doesn't need to handle `None`)
- simplified `PusherStep`
- signaling box movement is now done by single attribute, `moved_box_id`
- added explicit constant for "no piece ID is present" situations (`Config.NO_ID`)
- improved docs and tests, mainly on the front of exceptions
- fix: in many places we now check for negative values and raise early
- having ie. negative board width not only makes no sense, but is completely
impossible to pass into C++ layer where board with type is strictly defined and
is using unsigned integer

- massive refactoring and cleanup or public API
- updated .sok format reader to v0.19
- cleanly separated I/O from game engine (both architecturally and in implementation)
- `VariantBoard` monstrosity is gone
- Tessellation selection via inelegant and unsafe strings is gone
- replaced hand written parsers with Boost.X3 on C++ side and Lark on Python side
- implemented parts of I/O that were missing on C++ side
- Python doesn't really need I/O speedup here, but it was missing part of
libsokoengine C++
- consequently, to be able to test C++ I/O implementation, it was exported into Python
and is used alongside other parts of native extension

- improved build process for Python native extension