Idea: `suffix` and `stem` file columns

[simonw]

For https://sqlite-utils.datasette.io/en/stable/cli.html#inserting-data-from-files

Given a file called dogs.jpg stem would be dogs and ext would be jpg.

Need to decide what happens for dogs.and.cats.jpg.gz.

[simonw]

Pathlib says the stem of that would be dogs.and.cats.jpg - best stick with that for consistency. https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.suffix

It calls the last bit suffix - maybe I should use that instead of ext.