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/wasm-lib/kcl-macros/tests/macro_test.rs

@@ -1,7 +1,7 @@
 extern crate alloc;
 use kcl_lib::ast::types::{
-    BodyItem, Expr, Identifier, Literal, LiteralValue, NonCodeMeta, Program, VariableDeclaration, VariableDeclarator,
-    VariableKind,
+    BodyItem, Expr, Identifier, ItemVisibility, Literal, LiteralValue, NonCodeMeta, Program, VariableDeclaration,
+    VariableDeclarator, VariableKind,
 };
 use kcl_macros::parse;
 use pretty_assertions::assert_eq;
@@ -33,6 +33,7 @@ fn basic() {
                 })),
                 digest: None,
             }],
+            visibility: ItemVisibility::Default,
             kind: VariableKind::Const,
             digest: None,
         })],