org-element: Be stricter when matching arguments in LaTeX environments

* lisp/org-element.el (org-element--current-element): Be stricter when
  matching arguments in LaTeX environments.  In particular, do not
  allow anything else than options and arguments in the opening line.
* testing/lisp/test-org-element.el: Add tests.

lisp/org-element.el

@@ -3775,7 +3775,8 @@ element it has to parse."
 	      (goto-char (car affiliated))
 	      (org-element-keyword-parser limit nil))
 	     ;; LaTeX Environment.
-	     ((looking-at "[ \t]*\\\\begin{\\([A-Za-z0-9*]+\\)}")
+	     ((looking-at
+	       "[ \t]*\\\\begin{[A-Za-z0-9*]+}\\(\\[.*?\\]\\|{.*?}\\)*[ \t]*$")
 	      (org-element-latex-environment-parser limit affiliated))
 	     ;; Drawer and Property Drawer.
 	     ((looking-at org-drawer-regexp)

testing/lisp/test-org-element.el

@@ -1136,11 +1136,19 @@ e^{i\\pi}+1=0
        :value
        (org-element-map
 	   (org-element-parse-buffer) 'latex-environment 'identity nil t)))))
-  ;; Allow environments with options.
+  ;; Allow environments with options and arguments.
   (should
    (eq 'latex-environment
        (org-test-with-temp-text
 	   "\\begin{theorem}[Euler]\ne^{i\\pi}+1=0\n\\end{theorem}"
+	 (org-element-type (org-element-at-point)))))
+  (should
+   (eq 'latex-environment
+       (org-test-with-temp-text "\\begin{env}{arg}\nvalue\n\\end{env}"
+	 (org-element-type (org-element-at-point)))))
+  (should-not
+   (eq 'latex-environment
+       (org-test-with-temp-text "\\begin{env}{arg} something\nvalue\n\\end{env}"
 	 (org-element-type (org-element-at-point))))))