Add main module field

[aayla-secura]

See explanation in mesqueeb/copy-anything#28

--- a/is-what/package.json	2025-12-10 16:53:59.769712153 +0000
+++ b/is-what/package.json	2025-12-10 16:54:12.005707375 +0000
@@ -4,6 +4,7 @@
   "description": "JS type check (TypeScript supported) functions like `isPlainObject() isArray()` etc. A simple & small integration.",
   "type": "module",
   "sideEffects": false,
+  "main": "./dist/index.js",
   "exports": {
     ".": "./dist/index.js",
     "./*": "./dist/*"