Explicitly mark all packages with commonjs type

See: https://nodejs.org/api/packages.html#packages_determining_module_system

> Package authors should include the "type" field, even in packages where all sources are CommonJS. Being explicit about the type of the package will future-proof the package in case the default type of Node.js ever changes, and it will also make things easier for build tools and loaders to determine how the files in the package should be interpreted.

Related issue: adeira/universe#2341

adeira-source-id: 7288ec22447e6de87afa3b2c0d510eb52772ecb3

package.json

@@ -6,6 +6,7 @@
   "private": false,
   "version": "2.0.0",
   "main": "./src/SignedSource.js",
+  "type": "commonjs",
   "sideEffects": false,
   "dependencies": {
     "@babel/runtime": "^7.14.0"