Custom editors: implement proper untitled editor factory

Now that we have a untitled editor factory, custom editors should respect the untyped input that is passed in properly: * if no resource is given, generate a resource with a counter but keep track of editors and decrement once closed/disposed so that the counter does not just increase * if a associated resource is given, make sure to preserve that state [1] https://github.com/microsoft/vscode/blob/d0ca6c7003063cd07a85bef42eab61f92797de8a/src/vs/workbench/contrib/customEditor/browser/customEditors.ts#L131-L133 [1] untitled with associated resource * run from the command line `code <some path that does not exist for a custom editor file>` * expect the custom editor to open this file, showing the path and being dirty directly * expect saving to not prompt for a path but using that path Related issue for notebooks: https://github.com/microsoft/vscode/issues/125028