Add drag-and-drop support for displaying file contents in text editor

[zeevenn]

Now the text diff editor only supports copying or typing. Adding drag-and-drop support will improve user experience by allowing users to directly drop files into the viewer.

• Enable users to drag a file from their system and drop it onto the diff viewer area(left or right).
• Automatically read the file contents and display them.
• Support multiple text file types (e.g., .txt, .js, .json).
• Provide clear error messages for unsupported file types.

[zeevenn]

isBinaryFile may check text file type. But it doesn't support esm. And also have some edge cases: Assumes files with non-utf-8 encodings are binary

[gjtorikian]

👋 v6 now supports ESM and optional "encoding hinting"