Allow extensions to define new BlockTypes / ArgumentTypes

[tarehart]

Feature request:

It should be possible to write an extension that defines a custom ArgumentType and BlockType. For example, I want to define a Vector3 class that can be returned from reporters, passed into operators, and presented as a custom block shape.

Without this feature, people are forced to fork several packages to get the desired functionality, and cannot participate in the project sharing community.

[tarehart]

For my particular project, I ended up making significant changes to scratch-vm which would be beyond the scope of any reasonable extension. I don't know how many people would want to make custom block types but go no farther.

[glenpike]

This might be useful - just seen some extensions which have to do the same to the scratch-vm