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/tests/modify/main.rs

@@ -35,7 +35,7 @@ async fn setup(code: &str, name: &str) -> Result<(ExecutorContext, Program, uuid
     let parser = kcl_lib::parser::Parser::new(tokens);
     let program = parser.ast()?;
     let ctx = kcl_lib::executor::ExecutorContext::new(&client, Default::default()).await?;
-    let exec_state = ctx.run(&program, None, IdGenerator::default()).await?;
+    let exec_state = ctx.run(&program, None, IdGenerator::default(), None).await?;
 
     // We need to get the sketch ID.
     // Get the sketch ID from memory.