Refactor background page to use Typescript

Also, this CL adds clean-terminal-webpack-plugin to make it easier to
debug Typescript errors while developing the extension.

Bug: translateselectedtext:15
Change-Id: If9b97cb7508859e2e05f5dc82940808fd935bf1a
diff --git a/package.json b/package.json
index 4d38e49..2bda602 100644
--- a/package.json
+++ b/package.json
@@ -25,6 +25,8 @@
   "license": "MIT",
   "private": true,
   "dependencies": {
+    "chrome-types": "^0.1.113",
+    "clean-terminal-webpack-plugin": "^3.0.0",
     "clean-webpack-plugin": "^4.0.0",
     "copy-webpack-plugin": "^11.0.0",
     "json5": "^2.2.1",