oc-natbib: Provide a fallback bibliography style

* lisp/oc-natbib.el (org-cite-natbib-bibliography-style): New custom
option for default bibliography style.
(org-cite-natbib-export-bibliography): Use the new custom option.
* etc/ORG-NEWS (New ~org-cite-natbib-export-bibliography~ option
defining fallback bibliography style): Document the new option.

If we do not specify a bibliography style, LaTeX export will fail.
After the patch, the following simple-minded Org document will export
without errors:

 #+title: Testing org-cite \LaTeX export
 #+latex_header: \usepackage{natbib}
 #+bibliography: bibliography.bib
 #+options: toc:nil
 #+cite_export: natbib

 Hello World! This is a citation: [cite:@citationkey2023]

 #+print_bibliography:

etc/ORG-NEWS

@@ -24,6 +24,16 @@ consider [[https://gitlab.com/jackkamm/ob-python-mode-mode][ob-python-mode-mode]
 has been ported to.
 
 ** New and changed options
+*** New ~org-cite-natbib-export-bibliography~ option defining fallback bibliography style
+
+~natbib~ citation export processor now uses
+~org-cite-natbib-export-bibliography~ (defaults to ~unsrtnat~) as a
+fallback bibliography style if none is specified by user in
+=#+cite_export:= keyword.
+
+Previously, export would fail without explicitly selected bibliography
+style.
+
 *** New escape in ~org-beamer-environments-extra~ for labels in Beamer export
 The escape =%l= in ~org-beamer-environments-extra~ inserts the label
 obtained from ~org-beamer--get-label~.  This is added to the default

lisp/oc-natbib.el

@@ -77,6 +77,15 @@ If \"natbib\" package is already required in the document, e.g., through
     (const :tag "redefine \\thebibliography to issue \\section* instead of \\chapter*" sectionbib)
     (const :tag "keep all the authors' names in a citation on one line" nonamebreak)))
 
+(defcustom org-cite-natbib-bibliography-style 'unsrtnat
+  "Default bibliography style."
+  :group 'org-cite
+  :package-version '(Org . "9.7")
+  :type
+  '(choice
+    (const unsrtnat)
+    (symbol :tag "Other")))
+
 
 ;;; Internal functions
 (defun org-cite-natbib--style-to-command (style)
@@ -143,7 +152,9 @@ CITATION is the citation object.  INFO is the export state, as a property list."
   "Print references from bibliography FILES.
 FILES is a list of absolute file names.  STYLE is the bibliography style, as
 a string or nil."
-  (concat (and style (format "\\bibliographystyle{%s}\n" style))
+  (concat
+   (format "\\bibliographystyle{%s}\n"
+           (or style org-cite-natbib-bibliography-style))
    (format "\\bibliography{%s}"
            (mapconcat #'file-name-sans-extension
                       files