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_files.lua

@@ -5,7 +5,7 @@
 local util = require('gen.util')
 local fmt = string.format
 
-local DEP_API_METADATA = 'build/funcs_metadata.mpack'
+local DEP_API_METADATA = arg[1]
 local TEXT_WIDTH = 78
 
 --- @class vim.api.metadata