Enhance meson support so that it can automatically generate a test runner

[pmembrey]

The current meson.build file is all that's needed to link Unity into a parent project. However, this doesn't expose any of the helper scripts, with generate_test_runner.rb being one of the most important for "it just works" testing.

The added generator makes it easy to call generate_test_runner.rb with given input and output files, allowing Meson to automatically integrate the runner into its build environment. This did require making generate_test_runner.rb executable and adding a #! though.

The approach used is the best I can find from scouring the docs. From a brief discussion with a Meson dev, the path should magically do the right thing. Alas it doesn't and likely I'll need to file a bug. This works in the meantime however.

I've added some documentation in docs/ - happy to refine as needed :)