feat(examples): mirror realm (#3156)

<!-- please provide a detailed description of the changes made in this
pull request. -->

## Description

Adds the mirror realm.

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

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [x] Provided any useful hints for running manual tests
</details>

examples/gno.land/r/demo/mirror/doc.gno

@@ -0,0 +1,3 @@
+// Package mirror demonstrates that users can pass realm functions
+// as arguments to other realms.
+package mirror

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

@@ -0,0 +1,3 @@
+module gno.land/r/demo/mirror
+
+require gno.land/p/demo/avl v0.0.0-latest

examples/gno.land/r/demo/mirror/mirror.gno

@@ -0,0 +1,33 @@
+package mirror
+
+import (
+	"gno.land/p/demo/avl"
+)
+
+var store avl.Tree
+
+func Register(pkgpath string, rndr func(string) string) {
+	if store.Has(pkgpath) {
+		return
+	}
+
+	if rndr == nil {
+		return
+	}
+
+	store.Set(pkgpath, rndr)
+}
+
+func Render(path string) string {
+	if raw, ok := store.Get(path); ok {
+		return raw.(func(string) string)("")
+	}
+
+	if store.Size() == 0 {
+		return "None are fair."
+	}
+
+	return "Mirror, mirror on the wall, which realm's the fairest of them all?"
+}
+
+// Credits to @jeronimoalbi

examples/gno.land/r/leon/home/gno.mod

@@ -5,5 +5,6 @@ require (
 	gno.land/r/demo/art/gnoface v0.0.0-latest
 	gno.land/r/demo/art/millipede v0.0.0-latest
 	gno.land/r/demo/hof v0.0.0-latest
+	gno.land/r/demo/mirror v0.0.0-latest
 	gno.land/r/leon/config v0.0.0-latest
 )

examples/gno.land/r/leon/home/home.gno

@@ -9,6 +9,7 @@ import (
 	"gno.land/r/demo/art/gnoface"
 	"gno.land/r/demo/art/millipede"
 	"gno.land/r/demo/hof"
+	"gno.land/r/demo/mirror"
 	"gno.land/r/leon/config"
 )
 
@@ -34,6 +35,7 @@ TODO import r/gh
 	}
 
 	hof.Register()
+	mirror.Register(std.CurrentRealm().PkgPath(), Render)
 }
 
 func UpdatePFP(url, caption string) {