Update iron-list to use newer Templatize library

[rebekahpotter]

iron-list is still using the legacy Templatizer behavior from lib/legacy/templatizer-behavior.js, instead of the newer Templatize library (lib/utils/templatize.js).

This is requiring chromium to add an import of templatizer-behavior.js that would not otherwise be necessary, see https://chromium-review.googlesource.com/c/chromium/src/+/1832629.