Power Query doesn’t have syntax highlighting in its advanced editor (yet?). In the meantime, here is a language file for Notepad++. It’s not perfect, but I find it useful when writing M queries. It lists all of the current library functions as keywords (as of the PQ October update), recognizes comments, strings, literals, and language keywords.
You can find it on my One Drive share – here. You can find instructions on how to install a user defined language file in Notepad++ on their wiki site.
For more information on writing queries using the Power Query formula language, you can check out the deep dive session that Theresa Palmer and I gave at TechEd North America earlier this year.