Add lit components localization

Localization is performed via the @lit/localize package, which has been
integrated into the repo. A I18nLitElement class which extends
LitElement has been included, which automatically sets up the
localization.

This CL also introduces localization to the flattenThreads feature.

Fixed: twpowertools:157
Change-Id: If01540b9c8d70876a648aa002a1e0a5bede8f383
diff --git a/package.json b/package.json
index eb78dbb..3583b67 100644
--- a/package.json
+++ b/package.json
@@ -26,6 +26,7 @@
   ],
   "devDependencies": {
     "@babel/plugin-transform-modules-commonjs": "^7.20.11",
+    "@lit/localize-tools": "^0.6.7",
     "copy-webpack-plugin": "^9.0.1",
     "css-loader": "^6.2.0",
     "eslint": "^8.30.0",
@@ -44,6 +45,7 @@
   },
   "private": true,
   "dependencies": {
+    "@lit/localize": "^0.11.4",
     "@material/mwc-circular-progress": "^0.27.0",
     "@material/mwc-dialog": "^0.27.0",
     "@material/tooltip": "^12.0.0",