[meta] use npmignore to autogenerate an npmignore file

Author: ljharb

Diff for: .gitignore

@@ -32,3 +32,5 @@ node_modules
 npm-shrinkwrap.json
 package-lock.json
 yarn.lock
+
+.npmignore

Diff for: .npmignore

@@ -9,6 +9,7 @@
 	"license": "MIT",
 	"main": "index.js",
 	"scripts": {
+		"prepack": "npmignore --auto --commentLines=autogenerated",
 		"prepublishOnly": "safe-publish-latest",
 		"prepublish": "not-in-publish || npm run prepublishOnly",
 		"pretest": "npm run lint",
@@ -43,6 +44,7 @@
 		"in-publish": "^2.0.1",
 		"indexof": "^0.0.1",
 		"is": "^3.3.0",
+		"npmignore": "^0.3.0",
 		"nyc": "^10.3.2",
 		"safe-publish-latest": "^2.0.0",
 		"tape": "^5.6.1"
@@ -75,5 +77,11 @@
 		"commitLimit": false,
 		"backfillLimit": false,
 		"hideCredit": true
+	},
+	"publishConfig": {
+		"ignore": [
+			".github/workflows",
+			"test-corejs.js"
+		]
 	}
 }