I find writing in Twine directly really bad and prefer to copy paste from a text editor, so I wrote a user language for Notepad++ for SugarCube's syntax. It's pretty simple, focused on distinguishing text from code to make writing easier. Also has additional formatting features for different tags like bold to allow you to more easily spot them in the text.
The way this works is if you write syntax like <<if>> it will automatically format those into the size/color to distinguish them.
You can get it here: https://www.dropbox.com/s/hgesa8njefytu73/SugarCube.xml?dl=0
There should be a download button on the top right.
If you haven't used Notepad++ before, you can get it from here: http://notepad-plus-plus.org/
And you will also need to turn on "word wrap" from the view menu on first start. After that, go to settings, click "define your language", click "import", then choose the SugarCube.xml file.