feat(examples): add simplest "counter" realm (#2815)

I must have used this as an example a thousand times, but I just
realised it doesn't exist as an example.

<details><summary>Contributors' checklist...</summary>

- [ ] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

examples/gno.land/r/demo/counter/counter.gno

@@ -0,0 +1,14 @@
+package counter
+
+import "strconv"
+
+var counter int
+
+func Increment() int {
+	counter++
+	return counter
+}
+
+func Render(_ string) string {
+	return strconv.Itoa(counter)
+}

examples/gno.land/r/demo/counter/counter_test.gno

@@ -0,0 +1,22 @@
+package counter
+
+import "testing"
+
+func TestIncrement(t *testing.T) {
+	counter = 0
+	val := Increment()
+	if val != 1 {
+		t.Fatalf("result from Increment(): %d != 1", val)
+	}
+	if counter != val {
+		t.Fatalf("counter (%d) != val (%d)", counter, val)
+	}
+}
+
+func TestRender(t *testing.T) {
+	counter = 1337
+	res := Render("")
+	if res != "1337" {
+		t.Fatalf("render result %q != %q", res, "1337")
+	}
+}

examples/gno.land/r/demo/counter/gno.mod

@@ -0,0 +1 @@
+module gno.land/r/demo/counter