[meta] use npmignore to autogenerate an npmignore file

Author: ljharb

.gitignore

@@ -8,3 +8,5 @@ npm-shrinkwrap.json
 package-lock.json
 yarn-error.log
 yarn.lock
+
+.npmignore

.npmignore

@@ -17,6 +17,7 @@
   },
   "main": "lib/index.js",
   "scripts": {
+    "prepack": "npmignore --auto --commentLines=autogenerated",
     "build": "rimraf lib && babel src --out-dir lib --copy-files",
     "create": "node ./scripts/create-rule",
     "flow": "flow",
@@ -56,6 +57,7 @@
     "jscodeshift": "^0.7.1",
     "markdown-magic": "^2.6.1",
     "minimist": "^1.2.6",
+    "npmignore": "^0.3.0",
     "object.assign": "^4.1.2",
     "rimraf": "^3.0.2",
     "safe-publish-latest": "^2.0.0",
@@ -98,10 +100,16 @@
     ],
     "testEnvironment": "node"
   },
-  "greenkeeper": {
+  "publishConfig": {
     "ignore": [
-      "jest",
-      "babel-jest"
+      "!lib",
+      ".github/workflows",
+      ".flowconfig",
+      "/src",
+      "/reports",
+      "/flow",
+      "scripts/",
+      "CONTRIBUTING.md"
     ]
   }
 }