feat: Allow configuring the auto scroll edge offset

[snipd-min]

When using the MobileFloatingToolbar with a markdown editor not in full screen design (e.g. if a docked footer is present on the screen), the default auto scroll edge might not work and would lead to the following assertion error:

  assert(
      globalRect.size.width >= _dragTargetRelatedToScrollOrigin.size.width &&
          globalRect.size.height >=
              _dragTargetRelatedToScrollOrigin.size.height,
      'Drag target size is larger than scrollable size, which may cause bouncing',
    );

In this case it might be useful to allow configuring the edge offset.

[CLAassistant]

All committers have signed the CLA.

[LucasXu0]

@snipd-min Thanks for contribution. Please sign the CLA :)

[snipd-min]

Done :)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.40%. Comparing base (5d1d311) to head (4d0ad39). Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...ditor_component/service/scroll_service_widget.dart	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #888 +/- ## ========================================== - Coverage 72.40% 72.40% -0.01% ========================================== Files 317 317 Lines 14785 14787 +2 ========================================== + Hits 10705 10706 +1 - Misses 4080 4081 +1 ```

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