Be stricter on what's considered a scene path in CLI argument parsing

[Calinou]

Only consider the positional argument to be a scene path if it ends with a file extension associated with Godot scenes. This makes it possible for projects to parse command-line arguments for custom CLI arguments or other file extensions without trouble. This can be used to implement "drag-and-drop onto executable" logic, which can prove helpful for non-game applications.

This closes #8721.

[akien-mga]

Export now fails on CI:

ERROR: start: Command line param --export passed but no destination path given.

Please specify the binary's file path to export to. Aborting export.

   At: main/main.cpp:1500.

-------------------------------------------------------------------------

ERROR: No PCK file was created while exporting the project.

[lupoDharkael]

I've updated my parser PR based on this: b042d60

@Calinou Could you explain about the "drag-and-drop onto executable" line? I don't know what does that mean or how it is implemented. I may be relevant to my PR.

[Calinou]

@lupoDharkael What I mean here is that my PR makes drag-and-drop functionality possible (just like file associations, which also become possible). On most desktop OSes, when you drag and drop a file over an executable, it will be started with the absolute path to the file as an argument.

Of course, the application still needs to read command-line arguments, but it will no longer exit as soon as it's started because the scene path isn't valid.

[Calinou]

CLI exporting should be fixed now, if all goes well.

[akien-mga]

Thanks!