lang: Port TestAstFunc2 to txtar format

This ports TestAstFunc2 from our home-grown content storage system to
the txtar package. Since a single file can be used to represent the
entire folder hierarchy, this makes it much easier to see and edit
tests.

lang/interpret_test/TestAstFunc2/lambda-with-nested-poly-func.txtar

@@ -0,0 +1,13 @@
+-- main.mcl --
+import "fmt"
+
+$add = func($x) {
+	$x + $x
+}
+
+$num = 2
+$out = $add($num) # 4
+
+test fmt.printf("%d + %d is %d", $num, $num, $out) {} # simple math
+-- OUTPUT --
+Vertex: test[2 + 2 is 4]