It seemed to me at first that CodeMirror was overkill for editing library paths, but then I realized that some people use the "Edit" feature of FileManager to edit their .pg files.
We'll put this on the list of projects. I would be happy to accept a pull request (to the develop branch) if someone creates a version of FileManager that includes the necessary javaScript files for CodeMirror on the Edit and/or View tabs.