Allow different types of constants in Drasil

[B-rando1]

This is motivated by a discussion started in #2179, and summarized by @balacij's comment.

The core of the issue is that in programs, we call things constant for different reasons:

• There are some things we call constant because they represent something unchanging. Examples include gravity, database information, etc.
• There are some things we call constant because the flow of the program doesn't require that they change, so for safety or performance we say that they cannot change. Examples include intermediate calculations, e.g. const double discriminant = b*b - 4*a*c;, as the example @hrzhuang gave.
• There are other places where the semantics of constant and immutable are less clear - e.g. references, classes, methods.

I don't know when, but it would be good to have more flexibility of expression here. Here are my thoughts for what we need to do to get there:

• Slow down to do some more research and find a framework to organize our thinking on this.
• Look at Drasil, how it handles constants and immutable variables. We might only need to change drasil-code and drasil-gool, but it would be worthwhile to look higher than that too, e.g. to see if our chunks encode the information that we need.
• Start adding and using the necessary new functionality to enable the expressiveness we need.

If this is too much to do at once, we could start by limiting our scope to variables, not worrying yet about references, classes, etc.