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

src/gen/gen_eval.lua

@@ -1,10 +1,9 @@
 local mpack = vim.mpack
 
-local autodir = arg[1]
+local funcsfname = arg[1]
 local metadata_file = arg[2]
 local funcs_file = arg[3]
-
-local funcsfname = autodir .. '/funcs.generated.h'
+local eval_file = arg[4]
 
 --Will generate funcs.generated.h with definition of functions static const array.
 
@@ -48,7 +47,7 @@ hashpipe:write([[
 
 ]])
 
-local funcs = require('nvim.eval').funcs
+local funcs = loadfile(eval_file)().funcs
 for _, func in pairs(funcs) do
   if func.float_func then
     func.func = 'float_op_wrapper'