feat: add example for testing

examples/reference/test/nested.md

@@ -0,0 +1,22 @@
+# Out
+
+## See the `main fn`
+
+```rust
+// examples/nested/in.rs
+
+fn main() {
+    let stdout = stdout();
+    let message = String::from("Hello fellow Rustaceans!");
+    let width = message.chars().count();
+
+    let mut writer = BufWriter::new(stdout.lock());
+    say(&message, width, &mut writer).unwrap();
+}
+```
+
+## Take note of this special line
+
+```rust
+    let message = String::from("Hello fellow Rustaceans!");
+```

examples/test/nested.md

@@ -0,0 +1,15 @@
+# Out
+
+## See the `main fn`
+
+```rust
+// @?mainfn.file
+
+@@mainfn
+```
+
+## Take note of this special line
+
+```rust
+@@mainfnmessage
+```

examples/test/nested.rs

@@ -0,0 +1,15 @@
+use ferris_says::say;
+use std::io::{stdout, BufWriter};
+
+// @<mainfn
+fn main() {
+    let stdout = stdout();
+    // @<mainfnmessage
+    let message = String::from("Hello fellow Rustaceans!");
+    // >@
+    let width = message.chars().count();
+
+    let mut writer = BufWriter::new(stdout.lock());
+    say(&message, width, &mut writer).unwrap();
+}
+// >@