Add ExportPlugin::get_platform_os_name; use in C# export plugin

[raulsntos]

• Add ExportPlugin::get_platform_os_name method to retrieve the OS name for the current export's preset OS name from an export plugin without having to guess from the features.
• Use the new ExportPlugin::get_platform_os_name in the C# export plugin to get the platform name.
  • On its own it's not enough, but this may be of interest to @shana as part of making the export plugin more extensible to make it work with unknown platforms (i.e. consoles). For now the list of supported platforms is still hardcoded.

Alternative design

We could alternatively expose a method in ExportPlugin to retrieve the EditorExportPlatform and then we can just use the EditorExportPlatform::get_os_name that is already exposed.

[shana]

Yup, this will work out well enough for the consoles needs right now, where we really need a way for export plugins to know what platform they're currently exporting for without having to guess. I can work with this 👍

[akien-mga]

Alternative design

We could alternatively expose a method in ExportPlugin to retrieve the EditorExportPlatform and then we can just use the EditorExportPlatform::get_os_name that is already exposed.

Thinking about it, I think that could be useful. There's bound to be more platform-specific information we want to expose in the future so IMO it can make sense to start now with exposing EditorExportPlatform.

[raulsntos]

• Superseded by Allow adding custom export platforms using scripts / GDExtension. #90782.

I'll make a PR for the C# changes in this PR after #90782 is merged.