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/src/options/credits.json5 b/src/options/credits.json5
index 6572988..56220e2 100644
--- a/src/options/credits.json5
+++ b/src/options/credits.json5
@@ -17,6 +17,18 @@
     "license": "MIT License"
   },
   {
+    "name": "chrome-types",
+    "url": "https://github.com/GoogleChrome/chrome-types",
+    "author": "Google LLC",
+    "license": "Apache-2.0 License"
+  },
+  {
+    "name": "clean-terminal-webpack-plugin",
+    "url": "https://github.com/danillouz/clean-terminal-webpack-plugin",
+    "author": "Daniël Illouz",
+    "license": "MIT License"
+  },
+  {
     "name": "clean-webpack-plugin",
     "url": "https://github.com/johnagan/clean-webpack-plugin",
     "author": "John Agan",