Make pyproject bootstrap a separate command

[johnslavik]

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.

[johnslavik]

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

[jaraco]

Yes, that seems reasonable. Let's start with just coh bootstrap for now, which will simply write out the pyproject.toml, probably by calling __enter__() on write_pyproject() (and never calling __exit__). I'm tempted to say that it doesn't even need to take any parameters (let the caller clean it up with git clean -fx or delete it manually or commit it to their repo, as a lot of projects do now).