Export `EditorInspector::instantiate_property_editor` to use by plugins

[limbonaut]

This PR exports EditorInspector::instantiate_property_editor in the Godot API, so that plugin developers could reuse existing property editors in their projects. It also exports several useful properties and methods in the EditorProperty. Some of those properties are needed to implement custom inspectors (demo below).

See proposal: https://github.com/godotengine/godot-proposals/issues/8908

With this change, users can create a property editor like this:

EditorInspector.instantiate_property_editor(obj, TYPE_STRING, "some_property", PROPERTY_HINT_MULTILINE_TEXT, "", PROPERTY_USAGE_DEFAULT, false)

Test/demo project: Plugin that adds a custom inspector for a data object in the bottom panel: https://github.com/limbonaut/godot-custom-inspector-demo

[limbonaut]

I'll update it, and make a demo.

[limbonaut]

PR updated & rebased. And also updated the demo project, particularly fixed the issue with editing Packed*Array properties.

[Repiteo]

Thanks! Congratulations on your first merged contribution! 🎉