patch: add new sub-package wnb.stats to keep distribution-related modules

[msamsami]

• Add a new sub-package wnb.stats to contain distribution-related modules
• Move base.py to stats/base.py directory, add decorators to abstract methods in mixin classes
• Move enums.py and utils.py to stats/enums.py and stats/utils.py directories, respectively
• Move distribution classes from dist.py to stats/continuous.py and stats/discrete.py, keep them accessible from dist.py for backward compatibility
• Rename _typing.py to typing.py
• Rename DistibutionLike to DistributionLike (fix typo), move it from typing.py to stats/typing.py, keep it accessible from typing.py for backward compatibility
• Add necessary depreciation warnings
• Update imports in other scripts according to the changes above
• Bump version to 0.3.0