chore: move _embdemo to _demo

_demo/embed/esp32/hello/main.go

@@ -0,0 +1,16 @@
+package main
+
+import "github.com/goplus/lib/c"
+
+func myprint(s *c.Char) {
+	for i := 0; i < int(c.Strlen(s)); i++ {
+		WriteByte(byte(c.Index(s, i)))
+	}
+}
+
+func main() {
+	for {
+		myprint(c.Str("hello world"))
+		sleep(1)
+	}
+}