Make pyproject bootstrap a separate command

[bswck]

It would be very useful to triage what pyproject.toml is produced and allow maximum composability.
The install command essentially produces the pyproject.toml temporarily to just run a command with it.
However, I'd like to inspect the produced pyproject.toml to facilitate development & debugging.

@jaraco do you see this as worth implementing? I'll do it by hand.

[bswck]

I'm seeing it as a twofold command similar to those in git (like git rebase+git rebase --continue).

Imagine this:

coh bootstrap .  # produced the bootstrap files
<arbitrary commands>
coh bootstrap --clean .  # or any fitting naming