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/src/lit-locales/generated/.gitignore b/src/lit-locales/generated/.gitignore
new file mode 100644
index 0000000..afe8be7
--- /dev/null
+++ b/src/lit-locales/generated/.gitignore
@@ -0,0 +1,2 @@
+*.js
+*.ts
diff --git a/src/lit-locales/generated/README b/src/lit-locales/generated/README
new file mode 100644
index 0000000..39b1d0d
--- /dev/null
+++ b/src/lit-locales/generated/README
@@ -0,0 +1,5 @@
+This folder contains modules autogenerated by `make lit_localize_build`, ran by
+webpack when serving/building the extension.
+
+Please, do not add any file to this folder other than this document and the
+.gitignore file.