[meta] use npmignore to autogenerate an npmignore file

.gitignore

@@ -25,3 +25,5 @@ yarn.lock
 
 coverage/
 .nyc_output/
+
+.npmignore

package.json

@@ -9,6 +9,7 @@
 	"license": "MIT",
 	"main": "index.js",
 	"scripts": {
+		"prepack": "npmignore --auto --commentLines=autogenerated",
 		"version": "auto-changelog && git add CHANGELOG.md",
 		"postversion": "auto-changelog && git add CHANGELOG.md && git commit --no-edit --amend && git tag -f \"v$(node -e \"console.log(require('./package.json').version)\")\"",
 		"prepublishOnly": "safe-publish-latest",
@@ -69,6 +70,7 @@
 		"for-each": "^0.3.3",
 		"in-publish": "^2.0.1",
 		"make-arrow-function": "~1.1.0",
+		"npmignore": "^0.3.0",
 		"nyc": "^10.3.2",
 		"object.assign": "^4.1.4",
 		"safe-publish-latest": "^2.0.0",
@@ -96,17 +98,17 @@
 	"engines": {
 		"node": ">= 0.4"
 	},
-	"greenkeeper": {
-		"ignore": [
-			"core-js"
-		]
-	},
 	"auto-changelog": {
 		"output": "CHANGELOG.md",
 		"template": "keepachangelog",
 		"unreleased": false,
 		"commitLimit": false,
 		"backfillLimit": false,
 		"hideCredit": true
+	},
+	"publishConfig": {
+		"ignore": [
+			".github/workflows"
+		]
 	}
 }