The Advanced Editor in Power BI Desktop ... leaves something to be desired in terms of editing M queries. This utility can extract the M queries from a running copy of Power BI Desktop to a local path. Then the .m files can be edited offline using Visual Studio Code and the Power Query / M Language extension.
However, as far as I know, any changes made to the queries locally can't automatically be saved back to the Power BI model. So you are stuck pasting changes back into the application manually.
I would love to be proved wrong on this, of course.
Inspired by the blog post Using Visual Studio Code with Power BI.