[meta] use npmignore to autogenerate an npmignore file

Author: ljharb

.gitignore

@@ -31,3 +31,5 @@ node_modules
 npm-shrinkwrap.json
 package-lock.json
 yarn.lock
+
+.npmignore

.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",
@@ -50,6 +51,7 @@
 "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"
@@ -82,5 +84,11 @@
 "commitLimit": false,
 "backfillLimit": false,
 "hideCredit": true
+},
+"publishConfig": {
+"ignore": [
+".github/workflows",
+"test-corejs.js"
+]
 }
 }