src/examples/wasm: Show both methods supported

Adds another example showing the simple case
of executing main, adds a README explaining how
everything fits together and how to execute the compiled
code in the browser. Include a minimal webserver for
local testing.

src/examples/wasm/export/wasm.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"strconv"
+	"syscall/js"
+)
+
+func main() {
+}
+
+//go:export add
+func add(a, b int) int {
+	return a + b
+}
+
+//go:export update
+func update() {
+	document := js.Global().Get("document")
+	aStr := document.Call("getElementById", "a").Get("value").String()
+	bStr := document.Call("getElementById", "b").Get("value").String()
+	a, _ := strconv.Atoi(aStr)
+	b, _ := strconv.Atoi(bStr)
+	result := add(a, b)
+	document.Call("getElementById", "result").Set("value", result)
+}