Add easy customization for dir-locals files (Bug#66702)

Add easy customization for dir-locals files (Bug#66702)

* lisp/cus-edit.el (custom--editable-field-p): New utility function.
(custom-dirlocals-widget, custom-dirlocals-file-widget)
(custom-dirlocals-commands, custom-dirlocals-tool-bar-map): New
variables.
(custom-dirlocals-map, custom-dirlocals-field-map): New keymaps.
(Custom-dirlocals-menu): New menu.
(custom-dirlocals-key, custom-dynamic-cons, custom-dirlocals): New
widgets.
(custom-dirlocals-maybe-update-cons, custom-dirlocals-symbol-action)
(custom-dirlocals-change-file, custom-dirlocals--set-widget-vars)
(custom-dirlocals-get-options, custom-dirlocals-validate): New
functions.
(custom-dirlocals-with-buffer): New macro.
(Custom-dirlocals-revert-buffer, Custom-dirlocals-save)
(customize-dirlocals): New commands.

* doc/emacs/custom.texi (Directory Variables): Document
customize-dirlocals.

* etc/NEWS: Announce.