Typescript migration part 2 (#261)

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.

src/item-index.ts

@@ -20,12 +20,15 @@ class ItemIndex extends LitElement {
   query = "";
 
   @property({ type: Array })
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any -- TODO fixme
   filteredItems: any[] = [];
 
   @property({ type: Array })
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any -- TODO fixme
   sortedItems: any[] | null = null;
 
   @property({ type: Boolean })
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any -- TODO fixme
   hideHeader: any = null;
 
   @property({ type: String })
@@ -35,6 +38,7 @@ class ItemIndex extends LitElement {
   headerName = "Loaded Archives";
 
   @state()
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any -- TODO fixme
   private _deleting: any = {};
 
   private typeFilter = "";