To enable ligatures in Notepad++, go to Plugins > Plugins Admin... and find Luascript in the list of available plugins.
Enable and install it, then go to Plugins > Luascript > Edit Startup Script.
Notepad++ should open it up and the script will likely be empty or just about.
Add the following two lines to the end of the script:
editor1.Technology = SC_TECHNOLOGY_DIRECTWRITE
editor2.Technology = SC_TECHNOLOGY_DIRECTWRITEOnce done, simply restart Notepad++ and your changes should take effect. Remember, in order for this to work, you have to set the editor to use a font that supports ligatures.
Thank you to dail8859 for this solution.