chore: add unsigned keyword to number/uint32/base/add

The `unsigned` keyword is present in 8 of the 10 sibling `@stdlib/number/uint32/base/*` packages (80% conformance); `add` and `sub` were the only outliers. The function operates on unsigned 32-bit integers, so the keyword applies.

https://claude.ai/code/session_019SVGtKk9jEos5C9ecE4ni2

Author: claude

lib/node_modules/@stdlib/number/uint32/base/add/package.json

@@ -63,6 +63,7 @@
     "number",
     "uint32",
     "integer",
+    "unsigned",
     "c"
   ],
   "__stdlib__": {