feat: add abbreviation xs for ×ˢ, denoting the product set (#588)

Author: jhanschoo

lean4-unicode-input/src/abbreviations.json

@@ -1819,6 +1819,7 @@
     "X": "⨯",
     "vectorproduct": "⨯",
     "crossproduct": "⨯",
+    "xs": "×ˢ",
     "coprod": "⨿",
     "sigmaobj": "∐",
     "xf": "×ᶠ",