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.

scripts/gen_vimdoc.py

@@ -134,6 +134,7 @@ CONFIG = {
             'ui.lua',
             'filetype.lua',
             'keymap.lua',
+            'fs.lua',
         ],
         'files': [
             'runtime/lua/vim/_editor.lua',
@@ -142,6 +143,7 @@ CONFIG = {
             'runtime/lua/vim/ui.lua',
             'runtime/lua/vim/filetype.lua',
             'runtime/lua/vim/keymap.lua',
+            'runtime/lua/vim/fs.lua',
         ],
         'file_patterns': '*.lua',
         'fn_name_prefix': '',
@@ -167,6 +169,7 @@ CONFIG = {
             'ui': 'vim.ui',
             'filetype': 'vim.filetype',
             'keymap': 'vim.keymap',
+            'fs': 'vim.fs',
         },
         'append_only': [
             'shared.lua',