Feature request: Add `script` mode

[soronpo]

Currently scastie supports two modes: worksheet and non-worksheet. I propose changing it from boolean to enumeration of: worksheet, script, and app Under script mode, scastie will do them same as worksheet currently does, but without presenting values with the editor.

[OlegYch]

@soronpo i'm not sure why wouldn't you want to see the values in the editor

[soronpo]

For embedded scripts I mainly want to run. Only the console values are important and the values in the editor are just noises. So currently the application mode is more fitting but then it requires extending App which is also a "noise" when it comes to scripting.