Fix missed rename, and improve types (#276)

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.

src/pages.ts

@@ -165,9 +165,7 @@ class Pages extends LitElement {
       }
       const sorter = this.renderRoot.querySelector("wr-sorter") as Sorter;
       if (sorter) {
-        // @ts-expect-error - TS2339 - Property 'sortKey' does not exist on type 'Sorter'.
         sorter.sortKey = this.sortKey;
-        // @ts-expect-error - TS2339 - Property 'sortDesc' does not exist on type 'Sorter'.
         sorter.sortDesc = this.sortDesc;
       }
     }