Fix a type checking error related to `getFilterValues`

Fix a typechecking error in `useRootThread`.

`@return` has no effect when used above a variable initialized to a
value that is not an arrow function or function expression. Therefore
the type was inferred based on the `filterValues` local variable to be
`{}`. Correct this to `Object.<string,string>` so that it matches up
with the `ThreadState` type.

src/sidebar/store/modules/filters.js

@@ -250,12 +250,11 @@ function getFilter(state, filterName) {
 
 /**
  * Retrieve the (string) values of all currently-applied filters.
- *
- * @return {Object<string,string>}
  */
 const getFilterValues = createSelector(
   state => getFilters(state),
   allFilters => {
+    /** @type {Object.<string,string>} */
     const filterValues = {};
     Object.keys(allFilters).forEach(
       filterKey => (filterValues[filterKey] = allFilters[filterKey].value)