feat: add bundle:brave:beta script

This adds npm run bundle:brave:beta which outputs package into
build/brave/beta with the ID of the chrome beta channel

add-on/manifest.brave-beta.json

@@ -0,0 +1,3 @@
+{
+  "key": "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAshQhe/y/j0adZqOFes0nqgRgjDNx4eX6oNrESKeKbpUjH9eSN+lCYqBM3PT+BhPxo+sj/aVgCYsddiIbO43Bq/LsQLBFd+kD1I4qZSN4pJAX9AdsbMmXR9XV0W/O9zlyqkXAfxV13Hwmy+e6IH3p59ytQbpcuLnyipspQ4VXZprLkiWdvPMdifT9wgf5gmD30S1n7uaNrKCu8yZk/Lz5Z+KjoxRdk7X7FJYW+hoUGKb6Ld3Q99iLeKPIvcTjK6/xNHXTbaZfRYbfI8i/mSaxetGxSo7/XkMB8VvAiUkZ4gVSp786oMciQVwK2UyVFXw9pJhGD+O4ozcNk0PSq8aE7QIDAQAB"
+}

package.json

@@ -34,6 +34,7 @@
     "bundle:firefox": "shx cat add-on/manifest.common.json add-on/manifest.firefox.json | json --deep-merge > add-on/manifest.json && web-ext build -a build/firefox/",
     "bundle:firefox:beta": "shx cat add-on/manifest.common.json add-on/manifest.firefox.json add-on/manifest.firefox-beta.json | json --deep-merge > add-on/manifest.json && web-ext build -a build/firefox/beta/",
     "bundle:brave": "shx cat add-on/manifest.common.json add-on/manifest.brave.json | json --deep-merge > add-on/manifest.json && web-ext build -a build/brave/",
+    "bundle:brave:beta": "shx cat add-on/manifest.common.json add-on/manifest.brave.json add-on/manifest.brave-beta.json | json --deep-merge > add-on/manifest.json && web-ext build -a build/brave/beta/",
     "watch": "npm-run-all build:copy --parallel watch:*",
     "watch:js": "run-p watch:js:*",
     "watch:js:webpack": "webpack --watch --progress -d --devtool inline-source-map  --config ./webpack.config.js",