[Enhancement] Inline variable value hints

[Antyos]

This is a dedicated thread to discuss the enhancement proposed by @Dorro101 in #32.

Proposal

It would be convenient to have a toggleable feature that displays the evaluated value of variables inline. For example,

// Variables and arrays
width = 32;  // width = 32
height = 8;  // height = 8
clearance = 0.2;  // clearance = 0.2
size = [width, height + clearance];  // size = [32, 8.2]
name = "foo";  // name = "foo"

Note that what are displayed as "comments" are visual only and do not reflect the actual content of the file.

There are two parts to this:

1. The overlays themselves should be relatively straight forward to implement. The VSCode Decorator API can be used for this. See: vscode.rocks/decorations and microsoft/vscode-extension-samples@main/decorator-sample/USAGE.md
2. Evaluating the actual variables. This is the hard part because it requires a parser/interpreter. It is possible this will be easier with the implementation of a language server in https://github.com/Antyos/vscode-openscad/pull/31, but I am not too familiar with all of that.

Other questions that still need to be answers:

• How should for loops be treated?

  for (i = [0:5]) {  // i = [0, 1, 2, 3, 4]
      // Stuff
  }
  

• Or let() statements?
• Nesting is probably going to be an issue.

  for (i = [0:10]) {
      for (j = [0:i]) {
          for (k = [i:j]) {
              // Stuff
          }
      }
  }
  

• What about named parameters in function calls?

  cylinder(h = height, r1 = 10, r2 = r+10, center = true);  // h = 10, r1 = 10, r2 = 20, center = true
  

• How should long arrays or lots of parameters be handled?

Plans

As of right now, I have no immediate plans to implement this feature. However, I think it would be a cool feature to have in the extension. If anyone wants to take a go at this and submit a PR, please be my guest.

And an important question, does the nesting issue make this completely infeasible?