style: align string/base/slice-code-points with namespace conventions

Added missing `stdstring`, `utility`, and `util` keywords (present in 96-98% of `string/base/*` siblings).

Author: claude

lib/node_modules/@stdlib/string/base/slice-code-points/package.json

@@ -50,6 +50,7 @@
   ],
   "keywords": [
     "stdlib",
+    "stdstring",
     "string",
     "str",
     "base",
@@ -60,6 +61,8 @@
     "point",
     "codepoint",
     "utilities",
-    "utils"
+    "utility",
+    "utils",
+    "util"
   ]
 }