Document Length property for Dynamic Arrays

[ericoporto]

TODO: figure out where to add this!

The Length read-only property has been added to AGS 3.6.2.x with its legacy compiler. It has been added to the autocomplete in the editor too, in the script editor. We need to document it in the manual.

I am not sure where it should be added though, so I welcome suggestions. I think besides wherever is the reference place it is added, we will need to also mention it in a Upgrading to 3.6.2 entry, while it doesn't impact anything before it, it seems like a relevant new feature to mention in such a section.

[ivan-mogilko]

I think https://github.com/adventuregamestudio/ags-manual/wiki/DynamicArrays is a place to add this.