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-lock.json b/package-lock.json
index 6caff14..59a8ef6 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9,6 +9,8 @@
       "version": "0.0.0",
       "license": "MIT",
       "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",
@@ -516,6 +518,19 @@
         "node": ">=6.0"
       }
     },
+    "node_modules/chrome-types": {
+      "version": "0.1.113",
+      "resolved": "https://registry.npmjs.org/chrome-types/-/chrome-types-0.1.113.tgz",
+      "integrity": "sha512-vzon6Gcdtbzd7UJBa3Mwa2CFjtu7SV9jnejj3KMgYPbzHr+i1LAMAUA/uYi2n5z3mclEkdaXRlMRQHsE0C7c7g=="
+    },
+    "node_modules/clean-terminal-webpack-plugin": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/clean-terminal-webpack-plugin/-/clean-terminal-webpack-plugin-3.0.0.tgz",
+      "integrity": "sha512-wcgkQZmwEWYYjHblXc0+UGFDtx37S+1qgUQl4EOhhinzSHbZpixWBiasQ91RoCMf5lAm67j1XOt9z+HN+sWkWA==",
+      "peerDependencies": {
+        "webpack": "^4.0.0 || ^5.0.0"
+      }
+    },
     "node_modules/clean-webpack-plugin": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/clean-webpack-plugin/-/clean-webpack-plugin-4.0.0.tgz",
@@ -2577,6 +2592,17 @@
       "resolved": "https://registry.npmjs.org/chrome-trace-event/-/chrome-trace-event-1.0.3.tgz",
       "integrity": "sha512-p3KULyQg4S7NIHixdwbGX+nFHkoBiA4YQmyWtjb8XngSKV124nJmRysgAeujbUVb15vh+RvFUfCPqU7rXk+hZg=="
     },
+    "chrome-types": {
+      "version": "0.1.113",
+      "resolved": "https://registry.npmjs.org/chrome-types/-/chrome-types-0.1.113.tgz",
+      "integrity": "sha512-vzon6Gcdtbzd7UJBa3Mwa2CFjtu7SV9jnejj3KMgYPbzHr+i1LAMAUA/uYi2n5z3mclEkdaXRlMRQHsE0C7c7g=="
+    },
+    "clean-terminal-webpack-plugin": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/clean-terminal-webpack-plugin/-/clean-terminal-webpack-plugin-3.0.0.tgz",
+      "integrity": "sha512-wcgkQZmwEWYYjHblXc0+UGFDtx37S+1qgUQl4EOhhinzSHbZpixWBiasQ91RoCMf5lAm67j1XOt9z+HN+sWkWA==",
+      "requires": {}
+    },
     "clean-webpack-plugin": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/clean-webpack-plugin/-/clean-webpack-plugin-4.0.0.tgz",