[web] Fix multiline input selection in Chrome. (#168217)

Fixes https://github.com/flutter/flutter/issues/167805
Fixes https://github.com/flutter/flutter/issues/162698

### Description

First of all, I would like to thank @jezell for posting their fix of
this selection issue:
https://github.com/singerdmx/flutter-quill/issues/2450#issuecomment-2754652065

The issue with selection is happening because in Chrome `pointermove`
event and its coalesced events have some different targets. @mdebbar
already spotted this behavior some time ago and even filed a Chrome bug:
https://github.com/flutter/engine/pull/56949#discussion_r1871750266

This jsfiddle allows reproducing the bug:
https://jsfiddle.net/knevercode/y2hpfmrb/2/

On the following recording, you can see the events' targets and their
bounding boxes.
<video
src="https://github.com/user-attachments/assets/82af8d86-2cfe-4e36-a977-46ffa58facdb"/>

Those coalesced events have their `offsetX` and `offsetY` values
relative to the dummy `div` target. So to fix that, we have to translate
those values to be relative to the actual target.

This PR does exactly this in `_computeOffsetForInputs` when
`event.target != eventTarget` .

| Before | After |
| - | - |
| https://chrome-input-selection-bug.web.app |
https://chrome-input-selection-fix.web.app |
| <video
src="https://github.com/user-attachments/assets/1e10b43a-ff4f-46e8-8977-ee435f9d78fb"/>
| <video
src="https://github.com/user-attachments/assets/a997bfd0-8361-4bb3-910c-58cfc6b5d426"/>
|

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we expect every widget to implement].
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- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

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