Add \sqrt and \sqrt[3] as Latex input methods

* lisp/leim/quail/latin-ltx.el: Add \sqrt and \sqrt[3] as input
methods (bug#25594).

Author: larsmagne

lisp/leim/quail/latin-ltx.el

@@ -703,7 +703,8 @@ system, including many technical ones.  Examples:
  ("\\ldq" ?\“)
  ("\\rdq" ?\”)
  ("\\defs" ?≙)				; per fuzz/zed
- ;; ("\\sqrt[3]" ?∛)
+ ("\\sqrt" ?√)
+ ("\\sqrt[3]" ?∛)
  ("\\llbracket" ?\〚) 			; stmaryrd
  ("\\rrbracket" ?\〛)
  ;; ("\\lbag" ?\〚) 			; fuzz