build: add shorthands for interacting with the build system

We have a bootstrapping problem on most operating systems because the Poetry build system is not available as a distribution package. This is a problem for some users because it hinders developer productivity and automation.

This introduces a Makefile wrapper that installs Poetry into a venv (under ./venv/ by default, or configurable with the VENV environment variable). Various tasks can be added to this Makefile so one can easily interact with the build system without first having to activate the venv. This is useful for development environments where you do not have access to an interactive terminal.

It is important to note that the Makefile is not actually a dependent in the build process. Using poetry build, or any compatible Python build system, should still work. The Makefile is, then, merely a set of shorthand helpers to make development somewhat more enjoyable.