Don't fold blank lines at the end of the file
In the old TextEdit::fold_line function, blank lines at the end of the source file would be hidden. I rewrote the folding code so that only blank lines between non-blank lines in the indent block are folded.
Fixes #13339 (closed)
My apologies, when I made this PR, I did not think to check that someone else already fixed the problem. (https://github.com/godotengine/godot/pull/13362 does). However, that pull request makes some API changes, so I decided to make this pull request anyway. I accept the fault for not checking and don't mind if the maintainers choose the other PR over mine.