Calling editor.setSelectedText in system test

Our team is updating our Rails 7.0.8 app to Trix 2.0.7 from 1.3.1, which introduced a failure in one of our system tests that calls `setSelectedRange`:

```rb
content = "Hello world"
fill_in_rich_text_area with: content
find(:rich_text_area).execute_script("this.editor.setSelectedRange(#{content.length})")
```

On 1.3.1, this worked as expected; the text area was filled and then selection was moved to the end. On 2.0.7, the `setSelectedRange` call doesn't work; the selection stays at [0,0].

I haven't been able to find the root cause, and haven't found a way to get `setSelectedRange` working, but replacing `fill_in_rich_text_area` with Capybara's `set` method works around the issue:

```rb
content = "Hello world"
find(:rich_text_area).set(content)
```

I'm curious if there's a known change to `loadHTML` (called by [`fill_in_rich_text_area`](https://github.com/rails/rails/blob/fc734f28e65ef8829a1a939ee6702c1f349a1d5a/actiontext/lib/action_text/system_test_helper.rb#L33)) or `setSelectedRange` in 2.x that might have caused this behavior, but I understand this isn't necessarily a Trix bug, so feel free to close this as you see fit. I mainly wanted to note the issue and workaround for anyone else who might encounter it.

##### Details

* Trix version: 2.0.7
* Browser name and version: Chrome 119.0.6045.159 (via system test)
* Operating system: macOS 14.1

Start with actiontext/lib/action_text/system_test_helper.rb at the fill_in_rich_text_area entry point, then reproduce the Trix 2.0.7 behavior in the shown Chrome system test using setSelectedRange and loadHTML. Compare the selection result with Capybara's set workaround; done means identifying and fixing the regression or documenting that the workaround is required.

Written by the indexing model from the issue text.