Add tubeless ventil adapter

tl_ventil_adapter.scad

@@ -0,0 +1,11 @@
+difference() {
+    difference() {
+        cylinder(r1=4.2, r2=4.2, h=5, $fn=50);
+        cylinder(r1=3, r2=3, h=5, $fn=50);
+    }
+    translate([-5,0,10]) {
+        rotate([0,90,0]) {
+            cylinder(r1=7, r2=7, h=10, $fn=50);
+        }
+    }
+}