feat(build): build.zig MVP: build and run functionaltests on linux

NEW BUILD SYSTEM!

This is a MVP implementation which supports building the "nvim" binary,
including cross-compilation for some targets.
As an example, you can build a aarch64-macos binary from
an x86-64-linux-gnu host, or vice versa

Add CI target for build.zig currently for functionaltests on linux
x86_64 only

Follow up items:

-  praxis for version and dependency bumping
-  windows 💀
-  full integration of libintl and gettext (or a desicion not to)
-  update help and API metadata files
-  installation into a $PREFIX
-  more tests and linters

cmake/RunTests.cmake

@@ -68,7 +68,7 @@ endif()
 execute_process(
   # Note: because of "-ll" (low-level interpreter mode), some modules like
   # _editor.lua are not loaded.
-  COMMAND ${NVIM_PRG} -ll ${WORKING_DIR}/test/lua_runner.lua ${DEPS_INSTALL_DIR} busted -v -o test.busted.outputHandlers.nvim
+  COMMAND ${NVIM_PRG} -ll ${WORKING_DIR}/test/lua_runner.lua ${DEPS_INSTALL_DIR}/share/lua/5.1/ busted -v -o test.busted.outputHandlers.nvim
     --lazy --helper=${TEST_DIR}/${TEST_TYPE}/preload.lua
     --lpath=${BUILD_DIR}/?.lua
     --lpath=${WORKING_DIR}/src/?.lua