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/fs.lua

@@ -0,0 +1,35 @@
+local M = {}
+
+--- Iterate over all the parents of the given file or directory.
+---
+--- Example:
+--- <pre>
+--- local root_dir
+--- for dir in vim.fs.parents(vim.api.nvim_buf_get_name(0)) do
+---   if vim.fn.isdirectory(dir .. "/.git") == 1 then
+---     root_dir = dir
+---     break
+---   end
+--- end
+---
+--- if root_dir then
+---   print("Found git repository at", root_dir)
+--- end
+--- </pre>
+---
+---@param start (string) Initial file or directory.
+---@return (function) Iterator
+function M.parents(start)
+  return function(_, dir)
+    local parent = vim.fn.fnamemodify(dir, ":h")
+    if parent == dir then
+      return nil
+    end
+
+    return parent
+  end,
+    nil,
+    start
+end
+
+return M