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/functional/vimscript/api_functions_spec.lua

@@ -192,7 +192,7 @@ describe('eval-API', function()
     local screen = Screen.new(40, 8)
 
     command('set ft=vim')
-    command('set rtp^=build/runtime/')
+    n.add_builddir_to_rtp()
     command('syntax on')
     insert([[
       call bufnr('%')