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/common/i18n.ts b/src/common/i18n.ts
new file mode 100644
index 0000000..38bb0ca
--- /dev/null
+++ b/src/common/i18n.ts
@@ -0,0 +1,3 @@
+// Helper function which serves as a shorter alias to the chrome.i18n.getMessage
+// method, specially useful inside lit templates.
+export const msg = chrome.i18n.getMessage;