Document reportError as an option for reporting in browsers

### Problem Statement

I think the global `reportError` function could be an idiomatic way to report errors to Sentry from JavaScript, if it is not too general for your use case. I don't find much talk about this function on the webs, but it has been available across major browsers for some time.

### Solution Brainstorm

The Capturing Errors and Events page of the docs could have a small section that mentions `reportError` as an alternative.