feat: add extern let declarations for JS FFI

Add support for `extern let name: Type` and `extern let name: Type = "jsName"`
syntax for declaring external JavaScript values. This follows the same pattern
as extern fn across all compiler passes: parser, typechecker, interpreter
(runtime error placeholder), JS backend (emits JS name directly without
mangling), formatter, linter, modules, and symbol table.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/interpreter.rs

@@ -1431,6 +1431,14 @@ impl Interpreter {
                 Ok(Value::Unit)
             }
 
+            Declaration::ExternLet(ext) => {
+                // Register a placeholder that errors at runtime (extern lets only work in JS)
+                let name = ext.name.name.clone();
+                self.global_env
+                    .define(&name, Value::ExternFn { name: name.clone(), arity: 0 });
+                Ok(Value::Unit)
+            }
+
             Declaration::Effect(_) | Declaration::Trait(_) | Declaration::Impl(_) => {
                 // These are compile-time only
                 Ok(Value::Unit)