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/basic-conditional-scoped-lambda.txtar

@@ -0,0 +1,16 @@
+-- main.mcl --
+$some_bool = false
+$fn = if $some_bool {
+	func($b) {
+		"hello"
+	}
+} else {
+	func($bb) {
+		"world"
+	}
+}
+
+$out = $fn(false)
+test $out {}
+-- OUTPUT --
+Vertex: test[world]