Refactor options page to use Typescript

Also, I've added and ran eslint, and fixed several minor issues accross
the Typescript codebase.

Bug: translateselectedtext:15
Change-Id: I8cfd67697f9bfb22f6de93b64fd750de66bab863
diff --git a/.eslintrc.js b/.eslintrc.js
new file mode 100644
index 0000000..4b5889d
--- /dev/null
+++ b/.eslintrc.js
@@ -0,0 +1,20 @@
+module.exports = {
+    "env": {
+        "browser": true,
+        "es2021": true
+    },
+    "extends": [
+        "eslint:recommended",
+        "plugin:@typescript-eslint/recommended"
+    ],
+    "parser": "@typescript-eslint/parser",
+    "parserOptions": {
+        "ecmaVersion": "latest",
+        "sourceType": "module"
+    },
+    "plugins": [
+        "@typescript-eslint"
+    ],
+    "rules": {
+    }
+}