[meta] use npmignore to autogenerate an npmignore file

.gitignore

@@ -61,3 +61,5 @@ typings/
 npm-shrinkwrap.json
 package-lock.json
 yarn.lock
+
+.npmignore

package.json

@@ -20,6 +20,7 @@
 	"license": "MIT",
 	"main": "index",
 	"scripts": {
+		"prepack": "npmignore --auto --commentLines=autogenerated",
 		"prepublishOnly": "safe-publish-latest",
 		"prepublish": "not-in-publish || npm run prepublishOnly",
 		"pretest": "npm run lint",
@@ -62,6 +63,7 @@
 		"for-each": "^0.3.3",
 		"has-strict-mode": "^1.0.1",
 		"in-publish": "^2.0.1",
+		"npmignore": "^0.3.0",
 		"nyc": "^10.3.2",
 		"object-inspect": "^1.12.2",
 		"safe-publish-latest": "^2.0.0",
@@ -91,9 +93,9 @@
 	"engines": {
 		"node": ">= 0.4"
 	},
-	"greenkeeper": {
+	"publishConfig": {
 		"ignore": [
-			"nyc"
+			".github/workflows"
 		]
 	}
 }