Basic open/save functionality (revised)

[tinyg]

Port nobecutan's changes from "Basic open/save functionality" to the latest version of Boop and then make the errors show in the status view.

Original pull request comments:

With this merge request I propose basic file handling functionality. Open any file type and try to parse it as String via String(contentsOf: pathUrl) – silently ignore the file if it cannot be parsed. This works also via CMD+O and by dragging the file on the app icon.
This addresses #256 and #227.

Save the content as file (via CMD+SHIFT+S). There is no notion of an open file, it's always "Save As…".
This addresses #278.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[luckman212]

This would be an awesome merge! ✨

I'd love to be able to set Boop as my default handler for the public.json UTI for quick manipulation of JSON files.

[JanStorm]

I just compiled your version of the code. Seems that this breaks the editor.
In my case (Macbook Pro M1 Max) the line numbers were on the right and i cannot see any text i tried to write:

(But its somehow there, because when pressing enter, the line numbers increases, feels like the editor moved to the right, out the window)