Improve the horizontal drop target line.

[pszczesniak]

Suggested merge commit message (convention)

Other (theme-lark): Improve the drag and drop target line.

Closes #14645.

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Additional information

One thing is not done - white outline for darken backgrounds.

CSS outlines and box shadows wont work cause we're using two elements combined triangle a pseudo element to imitate the arrow and the line (vertical or horizontal). Those CSS properties works on each of them - for example a shadow from the arrow covers the line.

2nd approach was to use filter: drop-shadow and this solution was looking promising (Chrome):

Screen.Recording.2023-09-07.at.15.37.25.mov

BUT it completely not working on Safari (i found that dragging elements with filter: drop-shadow issue is there for years):

Screen.Recording.2023-09-07.at.15.36.18.mov

Using ASCII/Unicode signs as the arrows also wasn't working as expected.

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Idea 💡

Maybe in future we can think about different solution/approach - get the background color of the editing area and based on this set some light/dark colors (it's different than the prefers-color-scheme). Simple example of what i have on my mind.

[Witoso]

FYI @dagdzi, we needed to cut the white outline for now.