restructure: move el compiler content into lang/

lang/tests/text/examples/join.el

@@ -0,0 +1,12 @@
+// join.el — list-of-string join with separator.
+fn run_test() -> String {
+    let parts: [String] = native_list_empty()
+    let parts = native_list_append(parts, "alpha")
+    let parts = native_list_append(parts, "beta")
+    let parts = native_list_append(parts, "gamma")
+    return str_join(parts, ", ")
+}
+
+fn main() -> Void {
+    println(run_test())
+}