make both native and browser runtime optional

test.cwa

@@ -0,0 +1,13 @@
+import "env.memory" memory(4);
+import "env.printString" fn print(i32);
+
+export fn upd() {
+}
+
+start fn start() {
+    print(0);
+}
+
+data 0 {
+    "Press " i8(0xe0) " and " i8(0xe1) " to adjust, " i8(0xcc) " to commit." i8(0)
+}