Copy and paste does not preserve the original indentation

[PaulNovodonSWM]

Observed in Firefox 77.

Reproduce steps:

1. Copy a selection of a formatted JSON (make sure it contains indentation for the enclosed children objects)
2. Go to a text editor (any would do; mine were BBEdit on Mac and Notepad++ on Win)
3. Paste the selection
4. Observe that the indentation is lost and all lines start from the beginning (see attached images)

Original JSON selection:

Pasted selection:

[lauriro]

Replacing css based indentation with text based indentation has been in todo list for some years.
#15