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/i18n-config.toml b/i18n-config.toml
index 8c02374..dd55467 100644
--- a/i18n-config.toml
+++ b/i18n-config.toml
@@ -1,11 +1,22 @@
 base_path = "."
 
+# NOTE: Keep in sync with lit-localize.json
 locales = [
+  "ar",
   "ca",
+  "de",
   "es",
-  "ru",
+  "fr",
+  "id",
+  "it",
+  "ja",
+  "ko",
+  "pl",
   "pt_BR",
-  "pt_PT",
+  "ru",
+  "th",
+  "tr",
+  "vi",
 ]
 
 branches = [
@@ -19,3 +30,6 @@
 [[paths]]
   reference = "cws/en/listing-description.lang"
   l10n = "cws/{android_locale}/listing-description.lang"
+
+[[paths]]
+  l10n = "src/lit-locales/source/{android_locale}.xlf"