Draft: build: add subproject for edid-decode

Add a subproject for edid-decode, so that developers don't need to manually checkout and setup edid-decode.

A few things are missing in Meson before this can be properly implemented:

• Subproject fallbacks for find_program(): https://github.com/mesonbuild/meson/pull/6472
• Programs and projects without a configure step in the unstable external project module: https://github.com/mesonbuild/meson/issues/12478