[xref.el] Add `xref-find-references` to `xref-prompt-for-identifier`

[Ray <emacsray@gmail.com>]

Currently, xref-find-references prompts for the identifier to look up,
because it is not listed in xref-prompt-for-identifier:

(defcustom xref-prompt-for-identifier '(not xref-find-definitions
                                            xref-find-definitions-other-window
                                            xref-find-definitions-other-frame)

It will be much core convenient to add xref-find-references in the list.

Many language servers now support finding references. The typical
usage of lsp-mode is to put point at some identifier and hit a
shortcut (by default M-?) to trigger xref-find-references. The prompt
is in many cases undesired.