Fixes a missed rename, in `src/item-index.ts`: I'd renamed `coll` to
`item` but missed updating it in the `update` method's
`changedProperties` map.
This PR fixes that, and adds better typing to all of the instances of
`changedProperties` by using
[`PropertyValues<this>`](https://lit.dev/docs/components/lifecycle/#typescript-types-for-changedproperties),
and implementing class properties for missing Lit properties.
Converts everything to typescript, adding `@ts-expect-error`s where
necessary. Also makes some progress converting elements to use
decorators, but we're not fully there yet — I think it'll make sense to
continue to do that in future PRs, rather than doing it all in one go,
just so that we can keep up with the dev branch.
Emma Segal-Grossman