Allow Array.entry access syntax

[Jowan-Spooner]

Array and Dictionary entries in autoloads or other objects can be accessed like this {Autoload.array["entry"]}.

However the syntax {Autoload.array.entry}, which gdscript users would expect to also work, is not implemented.

This was mainly done, because I wanted to get some sort of access working quickly and my time is limited. Array.entry is harder to differentiate from normal object member access.

If we could implement this, it would be cool.