Copy external file drops into Treemacs projects

[liang799]

Summary

• add a Treemacs mouse customization for external drag-and-drop file copying
• handle dropped local file URIs in treemacs-mode by prompting to copy them into the project at point
• keep non-Treemacs drops and disabled-copy behavior on Emacs' default DND opening path
• document the behavior and cover copy, collision, and fallback paths in tests

Validation

• Ran the full Buttercup suite: Ran 257 specs, 0 failed
• Byte-compiled the changed Elisp files with warnings as errors using a temporary output directory

Notes

This PR keeps the feature disabled only when treemacs-copy-files-by-external-drag-and-drop is set to nil. The default is enabled so dropping readable local files onto a Treemacs project mirrors IDE-style project import behavior while still asking before copying.