[x] Organise functionality into submodules, use core for what's currently in brainreg
[x] Update imports and paths to account for new structure
[x] Update napari entry points
[x] Seperate the CLI for consistency. I don't think it needs to go into brainglobe-workflows as the CLI only includes functionality from the brainreg API. See https://github.com/brainglobe/brainreg/issues/139.
[x] Update package metadata
[x] Update pyproject.toml to accommodate new package structure
[x] Ensure that napari submodule is optional
[x] Update README
[x] Move old README content to website if necessary
[x] Preserve necessary workflows from retired repositories
[x] Push new version to PyPI
[x] Since we're only adding new functionality, we don't need an awkward "weird package history" warning
Subtask of #33.
brainglobe/brainregbrainreg-napari, preserving historycorefor what's currently inbrainregpyproject.tomlto accommodate new package structurenaparisubmodule is optional