Use pyproject.toml for build configs

[jsstevenson]

Local installation of the library in its current state emits the following warning:

DEPRECATION: Legacy editable install of vrs_anvil_toolkit==0.2.0rc4 from file:///Users/jss009/code/vrs_anvil_toolkit (setup.py develop) is deprecated. pip 25.0 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at https://github.com/pypa/pip/issues/11457

It's simple enough to just define everything in a pyproject.toml file. We can consolidate what's currently in setup.py and the requirements files into one place.

[jsstevenson]

Ope! closed in #112