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/loader.ts

@@ -1,4 +1,4 @@
-import { LitElement, html, css } from "lit";
+import { LitElement, html, css, type PropertyValues } from "lit";
 import { wrapCss } from "./misc";
 
 import prettyBytes from "pretty-bytes";
@@ -258,7 +258,7 @@ You can select a file to upload from the main page by clicking the 'Choose File.
     }
   }
 
-  updated(changedProperties) {
+  updated(changedProperties: PropertyValues<this>) {
     if (
       (this.sourceUrl && changedProperties.has("sourceUrl")) ||
       changedProperties.has("tryFileHandle")