Add yarn support for installing unreleased rrweb as a dependency (#497)

* Use prepack instead of prepare for yarn support

* add prepare and prepack

for yarn v1 & v2 compatibility

* Create .npmignore

package.json

@@ -3,7 +3,8 @@
   "version": "0.9.14",
   "description": "record and replay the web",
   "scripts": {
-    "prepare": "npm run bundle",
+    "prepare": "npm run prepack",
+    "prepack": "npm run bundle",
     "test": "npm run bundle:browser && cross-env TS_NODE_CACHE=false TS_NODE_FILES=true mocha -r ts-node/register test/**/*.test.ts",
     "test:watch": "PUPPETEER_HEADLESS=true npm run test -- --watch --watch-extensions js,ts",
     "repl": "npm run bundle:browser && cross-env TS_NODE_CACHE=false TS_NODE_FILES=true ts-node scripts/repl.ts",