Add manner to see and modify export variables for autoload scripts

[Spartan322]

Related: #4993

Every so often I see addons that add autoloads (usually singletons) to my projects, and sometimes when I'm using them too, I find I'd also want to have accessible variables without necessarily modifying the script every time.

The biggest ways to currently sort of replicate this behavior is:

1. Create an easily accessible and quite visible setting variable, either in something obviously named like a settings.gd or in top-level script, (often for an addon) for which all your settings constant references refer to.
2. Add a ProjectSetting for each option and use that as though that's an exported variable.

However both of these have a bunch of QoL/UI issues to me, especially since addons don't necessarily need to mess with the editor interface:

1. For setting variables:
   1. It is required for the user to inspect the scripts, especially for documentation, to modify and understand the setting.
   2. It does not integrate with the editor at all, and even lacking a constant requires a plugin implementation which could clutter the UI.
2. For ProjectSettings:
   1. It requires managing the ProjectSetting manually through a plugin else it will clutter the project's settings or annoy the user. (less of an issue if you allow a default value but now you need to document this and instruct the user on adding the proper setting)
   2. It could potentially conflict with another project setting, even if a low chance, this is still a possible risk.
   3. It has no clear association with the autoload script.
   4. As far as I'm aware, you can't add documentation to project settings, or if you can its definitely not intuitive.

Both of these methods require intentional paradigms to implement that aren't enforced in any manner by the language which can cause misunderstanding or perhaps even poor implementations. (or otherwise constant boilerplate)

In essence what seems to make the most sense (until/unless #4993 is added) is for an autoload to detect export variables and display another button that operates much like the inspector but only for those with export variables. (or perhaps its always active and just acts like selecting a node in the inspector but I feel that at such a point that might as well be a full implementation of #4993 then)