Apply compose sequences to String keys

This is mostly useful for characters that do not fit on a single 16-bit
char.
Shift sequences for 𝕨𝕩𝕗𝕘𝕤 are added for illustration.

srcs/compose/shift.json

@@ -125,5 +125,14 @@
   "ॢ": "ॣ",
   "॒": "॑",
   "ॅ": "ॲ",
-  "ॉ": "ऑ"
+  "ॉ": "ऑ",
+
+  // Mathematical symbols
+  "\uD835": {
+    "\uDD68": "𝕎", // 𝕨 → 𝕎
+    "\uDD69": "𝕏", // 𝕩 → 𝕏
+    "\uDD57": "𝔽", // 𝕗 → 𝔽
+    "\uDD58": "𝔾", // 𝕘 → 𝔾
+    "\uDD64": "𝕊"  // 𝕤 → 𝕊
+  }
 }