project_panel: Add precise drag-and-drop for files onto folded directories

[0xtimsb]

Closes #19192

1. Changed the drag overlay of entries for better visibility of where to drop.
2. Folded directories (except for the last folded one) will be highlighted as drop targets.
3. The delimiter between folded directories prevents the directory highlight from losing focus and acts as part of the directory to avoid flickering.

This works just like VS Code does.

fold-drop.webm

Release Notes:

• Added precise drag-and-drop for files onto folded directories in the Project Panel.

[0xtimsb]

Fixed a small edge case. Sorry for pushing after opening the PR.

[JosephTLyons]

Fixed a small edge case. Sorry for pushing after opening the PR.

There's no issue with pushing after opening - the team does this frequently. We tend to open PRs very early and just push to them as we work on them.

[JosephTLyons]

I think this might close this issue as well:

• Cannot drag and drop files to parent directories of joined nested folders #22922

[SomeoneToIgnore]

🚀
Thank you.