feat(fs): add vim.fs.parents()

vim.fs.parents() is a Lua iterator that returns the next parent
directory of the given file or directory on each iteration.

runtime/lua/vim/_editor.lua

@@ -50,6 +50,7 @@ for k, v in pairs({
   keymap = true,
   ui = true,
   health = true,
+  fs = true,
 }) do
   vim._submodules[k] = v
 end