add meson build support

[benmkw]

please feel free not to merge

pros:

• personally feel like its more widely used than premake and may make it easier for contributors or people trying out the examples (my motivation)

cons:

• not needed
• maintenance
• header only anyway
• only tested on macos for now

[Nelarius]

Yeah, I think it should definitely be easier to try out the examples. I was planning on doing by refactoring the examples and moving them to something like imnodes_demo.cpp which could be enabled in an existing ImGui project. I'll create an issue for that. 👍