[SuperEditor][Web] Fix option + arrow key selection changes. Resolves #2415

Steps to reproduce the behavior:

1. Run Example app on Chrome
2. Place cursor at beginning of line
3. Use CMD + SHIFT + RIGHT ARROW to select until the end of line
4. Use SHIFT + ALT + LEFT ARROW to remove last word from selection

When doing this the caret moves to the beginning of the line:

This seems to be caused by IME deltas. I ran the example editor without any keyboard handlers and the issue also happens. It might be related to the same issue fixed in #2327, where the line breaks on the browser HTML input are different from Flutter.

Currently, we have a keyboard handler that sends the event back to the IME when pressing LEFT or RIGHT ARROW at the middle of the text. I modified this handler to not do that if ALT is pressed.

This PR adds tests for this, but to effectively test these web keyboard interactions we would need an integration test that runs on the browser.