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

[angelosilvestre]

[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:

Screen.Recording.2024-11-19.at.11.37.40.AM.mov

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.

Screen.Recording.2024-11-20.at.18.58.37.mov

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.