* meson: Add a Meson based build system
This is primarily intended to be useful for projects using Meson wishing
to consume cxxopts as a subproject. By hosting the files upstream users
get the benefit of automatic parity with the CMake based install
provided by an OS vendor or distribution.
The implementation attempts to mirror the CMake build as much as
possible, with the exception of generating the cmake-config files, which
Meson doesn't currently support. It uses a small python script to parse
the cxxopts.hpp header to extract the version, due to a concious design
decision of Meson to leave such complex logic to external scripting
languages like Python.
* CI: add basic Meson testing
I've tried to be a bit more minimal here than the CMake tests are, since
there's already a good cross section of testing there. For Meson, I just
want to touch test each of the major platforms to ensure that it works
* fix: Use non deprecated upload-artifact action version
* fix: Use branch name in build workflow, use latest checkout action, use cd bulid as job steps doesn't preserve current directories between themselves.
* fix: remove non-existent main branch, remove deprecated ubuntu-20, removed deprecated macos-11 and macos-12, setup g++-9 and g++-10 if not exists, use latest checkout action
