Enable reliable fetch() timeout handling

### Problem description

Currently, `fetch()` will reject with a `TypeError` with the message `Network request timed out`. The only way to detect this error kind is matching its message, which might unexpectedly change in the future.

### Expected behavior

Errors thrown on timeout should have a distinct type (e.g. `TimeoutError`) to enable reliable error handling.

### Environment

Tabris.js version: 3.x