Divorce JSON and KCL (#4436)

Removes JSON from the KCL object model. Closes https://github.com/KittyCAD/modeling-app/issues/1130 -- it was filed on Nov 27 last year. Hopefully I close it before its one year anniversary.

Changes:

- Removed the UserVal variant from `enum KclValue`. That variant held JSON data.
- Replaced it with several new variants like Number, String, Array (of KCL values), Object (where keys are String and values are KCL values)
- Added a dedicated Sketch variant to KclValue. We used to have a variant like this, but I removed it as an experimental approach to fix this issue. Eventually I decided to undo it and use the approach of this PR instead.
- Removed the `impl_from_arg_via_json` macro, which implemented conversion from KclValue to Rust types by matching the KclValue to its UserVal variant, grabbing the JSON, then deserializing that into the desired Rust type. 
- Instead, replaced it with manual conversion from KclValue to Rust types, using some convenience macros like `field!`

src/lang/wasm.ts

@@ -336,7 +336,7 @@ export class ProgramMemory {
    */
   hasSketchOrSolid(): boolean {
     for (const node of this.visibleEntries().values()) {
-      if (node.type === 'Solid' || node.value?.type === 'Sketch') {
+      if (node.type === 'Solid' || node.type === 'Sketch') {
         return true
       }
     }