Filter published files on npm (#5626)

* Filter published files on npm

For now not gzipped leaflet package is 23mb. There's weird 4mb file `.goutputstream-N6HL8X` and a lot other stuff which increases node_modules size. `files` field can fix this with specified whitelist.

* Remove npmignore

.npmignore

@@ -1,10 +0,0 @@
-node_modules
-.DS_Store
-tmp/**/*
-.idea
-.idea/**/*
-*.iml
-*.sublime-*
-_site
-coverage/
-dist/leaflet.zip

package.json

@@ -30,6 +30,11 @@
   },
   "main": "dist/leaflet-src.js",
   "style": "dist/leaflet.css",
+  "files": [
+    "dist",
+    "src",
+    "!dist/leaflet.zip"
+  ],
   "scripts": {
     "test-jake": "jake test",
     "test": "jake test",