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.

test/functional/lua/fs_spec.lua

@@ -0,0 +1,33 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local exec_lua = helpers.exec_lua
+local eq = helpers.eq
+local mkdir_p = helpers.mkdir_p
+local rmdir = helpers.rmdir
+local nvim_dir = helpers.nvim_dir
+local test_build_dir = helpers.test_build_dir
+
+before_each(clear)
+
+describe('vim.fs', function()
+  describe('parents()', function()
+    it('works', function()
+      local test_dir = nvim_dir .. '/test'
+      mkdir_p(test_dir)
+      local dirs = exec_lua([[
+        local test_dir, test_build_dir = ...
+        local dirs = {}
+        for dir in vim.fs.parents(test_dir .. "/foo.txt") do
+          dirs[#dirs + 1] = dir
+          if dir == test_build_dir then
+            break
+          end
+        end
+        return dirs
+      ]], test_dir, test_build_dir)
+      eq({test_dir, nvim_dir, test_build_dir}, dirs)
+      rmdir(test_dir)
+    end)
+  end)
+end)