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/lua_runner.lua

@@ -2,10 +2,10 @@ local platform = vim.uv.os_uname()
 local deps_install_dir = table.remove(_G.arg, 1)
 local subcommand = table.remove(_G.arg, 1)
 local suffix = (platform and platform.sysname:lower():find 'windows') and '.dll' or '.so'
-package.path = (deps_install_dir .. '/share/lua/5.1/?.lua;')
-  .. (deps_install_dir .. '/share/lua/5.1/?/init.lua;')
+package.path = (deps_install_dir .. '/?.lua;')
+  .. (deps_install_dir .. '/?/init.lua;')
   .. package.path
-package.cpath = deps_install_dir .. '/lib/lua/5.1/?' .. suffix .. ';' .. package.cpath
+package.cpath = deps_install_dir .. '/?' .. suffix .. ';' .. package.cpath
 
 local uv = vim.uv