Fix overriding _export_begin, _export_file and _export_end from GDExtension

[dsnopek]

Fixes #80593

In EditorExportPlatform, it's explicitly checking for a script before calling these virtual methods on EditorExportPlugin, which prevents them from working correctly when implemented from GDExtension (rather than script).

This PR changes the code to instead check using GDVIRTUAL_IS_OVERRIDDEN_PTR() which will check for both script or GDExtension implementations.

Another (possibly better) way this could have been implemented is by making the default implementations of the virtual _export_begin() etc functions do GDVIRTUAL_CALL() (calling into script or GDExtension), allowing the C++ EditorExportPlugins to override those with custom implementations. It's done this way in a number of other classes. However, when I tried to implement it, there were issues with the fact that the in-engine classes take HashSet<String> p_features whereas the script/GDExtension versions take PackedStringArray, and this would have been recreating the PackedStringArray a lot more times than the current structure. If we changed the in-engine C++ API to take PackedStringArray instead, this would not be a problem!

But for now I've gone with the simpler approach.

NOTE: Currently marked as a draft, because I haven't actually tested it yet! The issue doesn't have an MRP, and there's a bunch of setup to test, but I will do it as soon as I can.

[dsnopek]

I've just made a test project (attached below) and verified that this seems to fix the bug! Taking out of draft.

gdext-editor-plugin-test.zip

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.2.