Add prepare hint to javascript example (#6016)

Without this additional line, v8 trunk produces no output, and v8 11.3
produces a bunch of assembly with no `imull` instruction.

I don't actually know much about v8 or javascript, but based on [this
mailing list
post](https://groups.google.com/g/v8-users/c/xlx22gScRNs/m/yE2-boWsBQAJ)
and testing on godbolt.org, this works.

examples/javascript/default.mjs

@@ -3,6 +3,9 @@ function square(a) {
 	return result;
 }
 
+// Collect type information on next call of function
+%PrepareFunctionForOptimization(square)
+
 // Call function once to fill type information
 square(23);