Implement trim_final_newlines setting and functionality

[bitwise-aiden]

Closes: godotengine/godot-proposals#8867

Currently in a draft state as I need to figure out the best method for collapsing carets that fall outside the removed area of text.

This PR implements the trim_final_newlines setting to reduce the newlines at the end of file to the expected 1 newline. This puts the built-in script editor on parity with other editors like Visual Studio Code and helps reduce stray lines making their way into user repos.

Implementation

I've opted to remove all bar the last empty lines from the file, checking simply for length 0. It intentionally runs after the trim_trailing_whitespace as to consume lines that only contain whitespace characters as well. In the case that trim_trailing_whitespace_on_save is not enabled it will then trim all lines back to the line with whitespace, leaving that untouched with a single newline following.

This relies on the functionality added in #86978 for caret and selection support.

[kitbdev]

Yeah, the selection code is more complex than it needs to be at the moment.
Try using TextEdit::set_selection_mode(TextEdit::SelectionMode::SELECTION_MODE_NONE, check_line, 0, c) when changing the selection, so that the caret's selecting_line gets set to a not invalid line.

Or if #86978 is merged first, you wouldn't have to worry about carets or selections at all, the remove_text() would handle it.

[bitwise-aiden]

Or if #86978 is merged first, you wouldn't have to worry about carets or selections at all, the remove_text() would handle it.

That's awesome context @kitbdev! Thanks :D
I don't have a great deal of bandwidth to push this forward at the moment so I'll remove the caret editing part, test against your branch, and leave it until you've pushed.

[bitwise-aiden]

@kitbdev I've just tested against this change, it all looks good, the errors have gone away and any caret issues seem to be resolved too. Thank you for taking all that on 🙌🏻

[bitwise-aiden]

Since @kitbdev's PR has been merged #86978, this is now ready for final review.

I've rebased and tested the cases locally where this was errorring earlier (multicaret / multi line selection) and everything seems to be working well.

[bitwise-aiden]

Updated with Kitbdev's last comment. Ready for review.

[akien-mga]

Looks good to me.

[akien-mga]

Thanks!