Thunderbird 136 retires ChromeUtils.import - replace with importESModule

While completing ESMIfication, Firefox 136 removed the function ChromeUtils.import(). This means wherever jsm modules were imported, we now need to import the matching ESM modules instead. Also all own jsm modules must be converted to sys.mjs.

See also:

https://thunderbird.topicbox.com/groups/addons/T51f3be1c0e12e5fe