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

test/testutil.lua

@@ -422,6 +422,10 @@ function M.is_asan()
   return M.paths.is_asan
 end
 
+function M.is_zig_build()
+  return M.paths.is_zig_build
+end
+
 local tmpname_id = 0
 local tmpdir = os.getenv('TMPDIR') or os.getenv('TEMP')
 local tmpdir_is_local = not not (tmpdir and tmpdir:find('Xtest'))