This enables building with NumPy on Linux and macOS. Windows support
should be complete to, but is untested as of now and may need a few
tweaks. This contains:
- A set of `meson.build` files and related code generation script
  tweaks, header templates, etc.
- One CI job on Linux
- Basic docs on using Meson to build NumPy (not yet integrated in the
  html docs, it's too early for that - this is for early adopters right
  now).

The build should be complete, with the major exception of SIMD support.
The full test suite passes. See gh-22546 for the tracking issue with
detailed notes on the plan for switching NumPy to Meson as its build
system.

Co-authored-by: Stefan van der Walt <stefanv@berkeley.edu>

When using an in-place build, this function calls various git commands to get
the version number. Previously it was called three times, making 'import
numpy' take over 3 seconds on my machine (it could be even slower on a slow
filesystem). The code has been refactored to only call this function once in
versions.py. Now 'import numpy' on an in-place build only takes 500 ms, which
is still slower than importing the installed version, but that is expected
when using versioneer.

- Make versions of the form '1.22.0.dev0' valid for non-releases.
- Put empty match at end of groups instead of at the beginning.
- Require eol in match, do not allow trailing characters.

The new tags look like '1.21.0.dev0+98.gaa0453721f', where '98' is the
number of commits since the 1.21.0 branch was started and 'aa0453721f'.
The chosen form may be specified in the 'setup.cfg' file. This PR adds
two new files 'numpy/_version.py' and 'numpy/version.py'. The latter
is kept because it is part of the public API and is actually used by
some downstream projects, but it is no longer dynamically created.

See https://github.com/python-versioneer/python-versioneer/ for more
information.