In addition to `src/sidebar/util`, it was also necessary to update types in
`src/sidebar/host-config.js` and `src/sidebar/build-thread.js` as these
are used from `src/sidebar/util`.

In the process I also found a case where the `call` function in
`postmessage-json-rpc.js` was being called with the wrong arguments in
some cases. The `timeout` parameter was passed in place of the `params`
argument.

 - Add stubs type definitions for objects returned from the Hypothesis
   API in `src/types/api.js`

 - Add and fix sufficient JSDoc type declarations that all JS files in
   src/sidebar/util typecheck.

 - Explicitly ignore a small number of expressions that TypeScript
   cannot typecheck using `// @ts-ignore`

 - Change several expressions from `expr` to `/** @type {Type} */
   (expr)` to narrow types down in cases where TS can't automatically
   infer the type.