GDScript: Fix some export annotation issues

[dalexeev]

• Add missing static variable check for @export_custom.
• Move the @export_storage implementation into a separate method and add a comment about the reasons.
• Document that PROPERTY_USAGE_SCRIPT_VARIABLE flag is always added.
• Fix Editor Help displaying rough types for annotation arguments.
• Add autocompletion for @export_custom arguments.
• Resolve Incoherent Property hint_string with Array #89816 (don't add property_hint for PROPERTY_HINT_NONE).
• Fix class_name and usage for enum arrays, improve utils.notest.gd to track changes in tests.
• Add more test cases in export_variable.gd.

[vnen]

LGTM

[akien-mga]

Thanks!