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/nvim/lua/converter.c

@@ -661,7 +661,7 @@ static inline void nlua_create_typed_table(lua_State *lstate, const size_t narr,
 void nlua_push_String(lua_State *lstate, const String s, int flags)
   FUNC_ATTR_NONNULL_ALL
 {
-  lua_pushlstring(lstate, s.data, s.size);
+  lua_pushlstring(lstate, s.size ? s.data : "", s.size);
 }
 
 /// Convert given Integer to Lua number