Kanban-python

- Add `^D` besides `^C` as option to close app (on windows pwsh its `^Z`).
- App closes now on `KeyboardInterrupt` and `EOFError`

- Bug fix: On first use the kanban_boards folder was not created. And therefore Board creation failed

- Move to XDG Path convention,
utilize `platformdirs` to write the config file to `user_config_dir` and the task files
to `user_data_dir`.
- added constants.py file for constants like the above mentioned Paths
- added more Tests
- added `platformdirs` <4 dependency
- Updated the docs

- BUGFIX settings.scanner keys not capitalized
- New Image for Readme

- New `kanban scan` option to scan for ` TODO` entries or other patterns.
Check Docs for example and Usage.
- Bug Fix: Prevent ValueError, if active board is not in board_list (couldve happened
if active board was deleted.) Now gives you option to change to other board.
- Add config options for `kanban scan` functionality
- Updated Readme/Docs accordingly

- Moved the board specific `pykanban.json` files into a dedicated `kanban_boards` directory
in the `.kanban-python` directory under `<BOARDNAME>/pykanban.json`.
This allows centrally stored tasks and doesnt scatter multiple
`pykanban.json` files over your projects.
- Adjusted functions/tests accordingly to new structure
- limiting Namespace of new boardnames to alpha-numeric + `-_ ` due to folder creation
- added default option (active board selection) for board change
- updated docs/readme