Add to `TextEdit` END key state support

[pafuent]

Supersedes #60239 (based on the discussions on that PR and re implemented to support multiple carets) Closes https://github.com/godotengine/godot-proposals/issues/10930

When the caret is set to the end of a line through ui_text_caret_line_end an internal state is kept in order to always keep the caret at the end of each line while is being moved through the different lines of the text. Also add a unit test for CodeEdit to check if END key state is kept on folding. Also preventing a SIGEV when SEND_GUI_ACTION is called with a non existing action.