restructure: move el compiler content into lang/

lang/examples/hello.el

@@ -0,0 +1,8 @@
+// hello.el — The simplest Engram program.
+// Run with: el run hello.el
+
+fn greet(name: String) -> String {
+    return "Hello, " + name
+}
+
+let message: String = greet("World")