feat: add a second two sided abbreviation for ‖z‖ (#634)

We already have `norm`, but this matches the convention for other
"bracketed" abbreviations where `\<opening><closing>` produces
`<opening>$CURSOR<closing>`.

Author: Julian

lean4-unicode-input/src/abbreviations.json

@@ -13,6 +13,7 @@
     "f<>": "‹$CURSOR›",
     "f<<>>": "«$CURSOR»",
     "[--]": "⁅$CURSOR⁆",
+    "||||": "‖$CURSOR‖",
     "nnnorm": "‖$CURSOR‖₊",
     "norm": "‖$CURSOR‖",
     "floor": "⌊$CURSOR⌋",