[BUG] Issue with cross-platform EOL (line endings)

[maettu-this]

Moved here from notepad-plus-plus/notepad-plus-plus#16639. See:

Issue 1: CR LF gets appended on [Enter], even though [Unix (LF)] is detected.

And:

I think there should be a warning in this case, something like "EOL is detected as [Unix (LF)] but .editorconfig has crlf", when opening such a file. And I think there should also issue a warning when selecting [Unix (LF)] but .editorconfig has crlf.

Related to #54.

[xuhdev]

Related: We considered a end_of_line=native option but concluded it was a bad idea: editorconfig/editorconfig#226

But a warning is OK if possible. But how to display the warning in an uninstrusive way?