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/lua/comment_spec.lua

@@ -47,7 +47,8 @@ local setup_treesitter = function()
 end
 
 before_each(function()
-  clear({ args_rm = { '--cmd' }, args = { '--clean' } })
+  -- avoid options, but we still need TS parsers
+  clear({ args_rm = { '--cmd' }, args = { '--clean', '--cmd', n.runtime_set } })
 end)
 
 describe('commenting', function()