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/testnvim.lua

@@ -17,7 +17,9 @@ local sleep = uv.sleep
 --- Functions executing in the current nvim session/process being tested.
 local M = {}
 
-local runtime_set = 'set runtimepath^=./build/lib/nvim/'
+local lib_path = t.is_zig_build() and './zig-out/lib' or './build/lib/nvim/'
+M.runtime_set = 'set runtimepath^=' .. lib_path
+
 M.nvim_prog = (os.getenv('NVIM_PRG') or t.paths.test_build_dir .. '/bin/nvim')
 -- Default settings for the test session.
 M.nvim_set = (
@@ -34,7 +36,7 @@ M.nvim_argv = {
   'NONE',
   -- XXX: find treesitter parsers.
   '--cmd',
-  runtime_set,
+  M.runtime_set,
   '--cmd',
   M.nvim_set,
   -- Remove default user commands and mappings.
@@ -425,7 +427,7 @@ local function remove_args(args, args_rm)
       last = ''
     elseif vim.tbl_contains(args_rm, arg) then
       last = arg
-    elseif arg == runtime_set and vim.tbl_contains(args_rm, 'runtimepath') then
+    elseif arg == M.runtime_set and vim.tbl_contains(args_rm, 'runtimepath') then
       table.remove(new_args) -- Remove the preceding "--cmd".
       last = ''
     else