Add safe-local-variable-directories variable.

Add safe-local-variable-directories variable.

This variable can be set to automatically load risky dir-local
variables from a list of trusted directories.

* lisp/emacs-lisp/files.el (safe-local-variable-directories,
hack-local-variables-filter, hack-local-variables-confirm): New
variable and associated logic.
* test/lisp/files-tests.el
(files-tests-safe-local-variable-directories): Add tests for same.
* doc/emacs/custom.texi (Safe File Variables): Add documentation for
same.
* doc/lispref/variables.texi (File Local Variables): Add documentation
for same.
* etc/NEWS (Lisp Changes in Emacs 30.1): Add news entry for
same.  (Bug#61901)