handle edge case at the very last line of a file for "new_line_below"

[jsonjunkie]

kinda of mentioned in this issue #21 but when editing a file in the godot editor and the cursor is at the very last line, the "new_line_below" offset makes a new line above the current line

actual:

expected:

[jsonjunkie]

actually, im not sure if you want to merge this, im seeing it causes an error in the output (no other noticeable errors or crashes as a result)

scene/gui/text_edit.cpp:7824 - Index p_from_column = 28 is out of bounds (text[p_from_line].length() + 1 = 2).

it seems like there is some weirdness with the godot editor and the end of files in general