Recognize shebang lines that pass '-S/--split-string' to 'env'

* etc/NEWS: announce the change.

* lisp/files.el (auto-mode-interpreter-regexp): Add optional '-S'
switch to the ignored group capturing the env invocation.
Allow multiple spaces between #!, interpreter and first argument:
empirically, Linux's 'execve' accepts that.  (Bug#66902)

* test/lisp/files-tests.el (files-tests--check-shebang): New helper to
generate a temporary file with a given interpreter line, and assert
that the mode picked by 'set-auto-mode' is derived from an expected
mode.  Write the 'should' form so that failure reports include useful
context; for example:

    (ert-test-failed
     ((should
       (equal (list shebang actual-mode) (list shebang expected-mode)))
      :form
      (equal ("#!/usr/bin/env -S make -f" fundamental-mode)
	     ("#!/usr/bin/env -S make -f" makefile-mode))
      :value nil :explanation
      (list-elt 1 (different-atoms fundamental-mode makefile-mode))))

* test/lisp/files-tests.el (files-tests-auto-mode-interpreter): New
test; exercise some aspects of 'interpreter-mode-alist'.

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