**Summary**
This is the final step in adding pluggable chunking-strategies. It
introduces the `chunk()` function to replace calls to strategy-specific
chunkers in the `@add_chunking_strategy` decorator. The `chunk()`
function then uses a mapping of chunking-strategy names (e.g.
"by_title", "basic") to chunking functions (chunkers) to dispatch the
chunking call. This allows other chunkers to be added at runtime rather
than requiring a code change, which is what "pluggable" chunkers is.
**Additional Information**
- Move the `@add_chunking_strategy` to the new `chunking.dispatch`
module since it coheres strongly with that operation, but publish it
from `chunking(.__init__)` (as it was before) so users don't couple to
the way we organize the chunking sub-package. Also remove the third
level of nesting as it's unrequired in this case.
- Add unit tests for the `@add_chunking_strategy` decorator which was
previously uncovered by any direct test.
