Remove all uses of the deprecated Electron remote module and replace with more secure IPC implementation. This also cleanly decouples the GUI app from the code that accesses the file system, and should enable us to eventually run PanWriter in a browser as well.
convert all PureScript files (which weren't that many) and most JavaScript files (which unfortunately had become many) to TypeScript, based on this electron template
remove ability to read user-customizable css files (maybe we can add something like that in the future again, but will have to work with pandoc's document-css, which we'd need to parse properly)