Had to make various other changes due to incompatibilities.
- Use the new `lit` package instead of `lit-html` or `lit-element`
- Drop `haunted` since it breaks the rules by specifying `type: module`
but then doesn't import with file extensions
- Use Sass Dart instead of node-sass (fixes#2445)
- Upgrade Karma
Removed Jed from the headless build and instead let `__` call sprintf.
It's now up to downstream users for `@converse/headless` to decide
whether they want i18n support for the few translatable strings in that package
and to implement it themselves.
