chore(examples): add microposts example (#3660)

Signed-off-by: moul <[email protected]>

examples/gno.land/r/moul/microposts/README.md

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+# fork of `leon/fosdem25/microposts`
+
+removing optional lines to make the code more concise for slides.
+
+Original work here: https://gno.land/r/leon/fosdem25/microposts

examples/gno.land/r/moul/microposts/gno.mod

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+module gno.land/r/moul/microposts

examples/gno.land/r/moul/microposts/microposts_test.gno

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+package microposts
+
+// empty file just to make sure that `gno test` tries to parse the implementation.

examples/gno.land/r/moul/microposts/post.gno

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+package microposts
+
+import (
+	"std"
+	"time"
+)
+
+type Post struct {
+	text      string
+	author    std.Address
+	createdAt time.Time
+}
+
+func (p Post) String() string {
+	out := p.text + "\n"
+	out += "_" + p.createdAt.Format("02 Jan 2006, 15:04") + ", by " + p.author.String() + "_"
+	return out
+}

examples/gno.land/r/moul/microposts/realm.gno

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+package microposts
+
+import (
+	"std"
+	"strconv"
+	"time"
+)
+
+var posts []*Post
+
+func CreatePost(text string) {
+	posts = append(posts, &Post{
+		text:      text,
+		author:    std.PrevRealm().Addr(), // provided by env
+		createdAt: time.Now(),
+	})
+}
+
+func Render(_ string) string {
+	out := "# Posts\n"
+	for i := len(posts) - 1; i >= 0; i-- {
+		out += "### Post " + strconv.Itoa(i) + "\n" + posts[i].String()
+	}
+	return out
+}