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

CMakeLists.txt

@@ -241,7 +241,7 @@ set(STYLUA_DIRS runtime scripts src test contrib)
 add_glob_target(
   TARGET lintlua-luacheck
   COMMAND $<TARGET_FILE:nvim_bin>
-  FLAGS -ll ${PROJECT_SOURCE_DIR}/test/lua_runner.lua ${CMAKE_BINARY_DIR}/usr luacheck -q
+  FLAGS -ll ${PROJECT_SOURCE_DIR}/test/lua_runner.lua ${CMAKE_BINARY_DIR}/usr/share/lua/5.1 luacheck -q
   GLOB_DIRS runtime scripts src test
   GLOB_PAT *.lua
   TOUCH_STRATEGY PER_DIR)