GDScript: Add module description in markdown

[anvilfolk]

This adds a .md file to the GDScript module that gives a moderately in-depth description of how the module is organized, used, main classes, etc :)

[dalexeev]

Great job! I didn’t know much of this or wasn’t completely sure. I've added some suggestions, you don't have to follow them all, these are just ideas that might inspire you.

The README is intended for new contributors, so it's quite difficult to decide which details are important to understand the module and which will require learning the code anyway, so it's not worth the effort.

In my opinion, a section about tests would help new contributors. There is a doc page, but it is mainly about doctest, not about GDScript tests. It would be nice to briefly explain that tests should be positive and negative, how our files are located (folders, *.gd, *.notest.gd, *.out), how to run tests locally.

I hope Adam, Dmitry and George will add their suggestions too.

[anvilfolk]

@dalexeev excellent comments! I will try to find some time soon to incorporate them! I think a section about tests makes a lot of sense, and we can link to the official docs about it!

[anvilfolk]

Incorporated all of @dalexeev's suggestions! They were so helpful and relevant! 🥰

[akien-mga]

Amazing write-up, I learned a lot :)

[anvilfolk]

Amazing write-up, I learned a lot :)

Thanks, it's all I'm capable of doing at the moment, but it sounds like folks might find it helpful, so that feels nice :)

It might eventually make sense to add a section on how unit tests are run, but that requires digging more deeply into that code, which I haven't done in a while :)

[anvilfolk]

Spent a few more hours on this today. I think it's good to merge if folks don't find anything else they'd like changed.

Changelog:

• Fixed some references to _populate_class_members() which no longer exists and has been replaced with _prepare_compilation()
• Added a lot more information and examples to the analyzer, since it had very, very little information
• Added more information about how cyclic dependencies are resolved
• Added some information about how typing leads to optimizations during compilation
• Tiny edits here and there

I think it's helpful for this document to describe a few of the things the analyzer actually does. This means that new contributors don't need to work through a couple hundred lines of code to try to figure it out - they can read it and get a general idea for what it does, and then read the code with more context in mind.

[Mickeon]

May I poke this? It looks great to me. Would be nice to have sooner than later for the developers

[dalexeev]

Looks good to me. Of course, there is always more to add, but we are not trying to write exhaustive internal documentation. This is a good overview for new contributors, and we can edit it later anyway (unless @vnen, @adamscott, @vonagam, and other contributors have suggestions right now). @anvilfolk Thank you again for the great write-up!

[akien-mga]

Thanks! Great writeup :)