Replace line breaks in virtual text with whitespace (#3949)
Replace line breaks in virtual text with whitespace (#3949)

Before this patch multiline warnings would appear in a single line with
'^@' as separator.
Now we use whitespace as separator to improve the appearance.

Also strip trailing whitespace, newlines, etc...

Fixes #3939
w0rp/ale,IngoHeimbach/ale,IngoHeimbach/ale,w0rp/ale,IngoHeimbach/ale,w0rp/ale