internal: KCL modules, part 1 (#4149)

Addresses #4080. (Not ready to close it yet.)

# Important

Requires a fix for #4147 before it can work in ZMA.

# Overview

```kcl
// numbers.kcl
export fn inc = (x) => {
  return x + 1
}
```

```kcl
import inc from "numbers.kcl"

answer = inc(41)
```

This also implements multiple imports with optional renaming.

```kcl
import inc, dec from "numbers.kcl"
import identity as id, length as len from "utils.kcl"
```

Note: Imported files _must_ be in the same directory.

Things for a follow-up PR:

- #4147. Currently, we cannot read files in WebAssembly, i.e. ZMA.
- Docs
- Should be an error to `import` anywhere besides the top level. Needs parser restructuring to track the context of a "function body".
- Should be an error to have `export` anywhere besides the top level. It has no effect, but we should tell people it's not valid instead of silently ignoring it.
- Error message for cycle detection is funky because the Rust side doesn't actually know the name of the first file. Message will say "b -> a -> b" instead of "a -> b -> a" when "a" is the top-level file.
- Cache imported files so that they don't need to be re-parsed and re-executed.

src/lang/modifyAst.ts

@@ -501,6 +501,7 @@ export function sketchOnExtrudedFace(
       createIdentifier(extrudeName ? extrudeName : oldSketchName),
       _tag,
     ]),
+    undefined,
     'const'
   )
 
@@ -682,6 +683,7 @@ export function createPipeExpression(
 export function createVariableDeclaration(
   varName: string,
   init: VariableDeclarator['init'],
+  visibility: VariableDeclaration['visibility'] = 'default',
   kind: VariableDeclaration['kind'] = 'const'
 ): VariableDeclaration {
   return {
@@ -699,6 +701,7 @@ export function createVariableDeclaration(
         init,
       },
     ],
+    visibility,
     kind,
   }
 }