Have the `Create Environment` command ask users which requirements file to install

[brettcannon]

We should search the top-level workspace directory and one level down for any (.+-)?requirements(-.+)?.txt files as well as requirements directory at the top-level with any *.txt files. We can present a quick pick for people to choose which requirements they want installed (and we can default to something reasonable, although what "reasonable" is I'm not sure 😅).

[brettcannon]

We can pre-check options where we think it makes sense in the default case.