**Summary**
Eliminate historical "idiosyncracies" of `table.metadata.text_as_html`
HTML introduced by `partition_pptx()`. Produce minified `.text_as_html`
consistent with that formed by chunking.
**Additional Context**
- PPTX `.metadata.text_as_html` is minified (no extra whitespace or
thead, tbody, tfoot elements).
- `table.text` is clean-concatenated-text (CCT) of table.
- Last use of `tabulate` library is removed and that dependency is
removed from `base.in`.
**Summary**
In preparation for consolidating post-partitioning metadata decorators,
extract `partition.common` module into a sub-package (directory) and
extract `partition.common.metadata` module to house metadata-specific
object shared by partitioners.
**Additional Context**
- This new module will be the home of the new consolidated metadata
decorator.
- The consolidated decorator is a step toward removing post-processing
decorators from _delegating_ partitioners. A delegating partitioner is
one that convert its file to a different format and "delegates" actual
partitioning to the partitioner for that target format. 10 of the 20
partitioners are delegating partitioners.
- Removing decorators from delegating partitioners will allow us to
avoid "double-decorating", i.e. running those decorators twice, once on
the principal partitioner and again on the proxy partitioner.
- This will allow us to send `**kwargs` to either partitioner, removing
the knowledge of which arguments to send for each file-type from
auto-partition.
- And this will allow pluggable auto-partitioners which all have a
`partition_x(filename, *, file, **kwargs) -> list[Element]` interface.
