This integrates the "main" test suite (test/test.cc) in Meson.
This allows to run the tests in the CI with the Meson-built version of
the library to ensure that nothing breaks unexpectedly.
It also simplifies life of downstream packagers, that do not have to
write a custom build script to split the library and run tests but can
instead just let Meson do that for them.
* Full Meson support
cpp-httplib can be now built with Meson even in compiled library mode.
The library is built with LTO, supports OpenSSL, zlib and Brotli,
and the build system also generates a pkg-config file when needed.
Compared to the CMake file this one is quite small (more than five times
smaller!), and maintaining it won't be an issue :)
* meson: automatic versioning
