Generate simple app.psgi from dancer2 script. do() it from bin/app.pl

As noted in #374, generate `./app.psgi`, and revise the content of the generated `bin/app.pl` to add
"../lib" to @INC then do() the new `app.psgi` file. i.e. logic similar to the now deleted ModuleLoader->use_lib (@xsaywerx++) was added to `bin/app.pl`; it's a more sane place for those who prefer the inbuilt server over
plackup.