feat(body-reader): add example for mocket body reader implementation

Implement a basic example demonstrating how to create a custom body reader
for the mocket framework. Includes package configuration and main example
showing CSV body parsing functionality.

Author: oboard

src/examples/body-reader/main.mbt

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+///|
+using @mocket {type HttpResponse}
+
+///|
+struct Csv(String)
+
+///|
+impl @mocket.BodyReader for Csv with from_request(req) -> Csv {
+  let text : String = req.body()
+  Csv(text)
+}
+
+///|
+fn main {
+  let app = @mocket.new()
+
+  // Text Response
+  app.post("/", event => {
+    let csv : Csv = event.req.body() catch { _ => Csv("") }
+    HttpResponse::new(OK).body(csv.0)
+  })
+
+  // Print Server URL
+  for path in app.mappings.keys() {
+    println("\{path.0} http://localhost:4000\{path.1}")
+  }
+
+  // Serve
+  app.serve(port=4000)
+}

src/examples/body-reader/moon.pkg.json

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+{
+  "is-main": true,
+  "import": ["oboard/mocket"]
+}

src/examples/body-reader/pkg.generated.mbti

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+// Generated using `moon info`, DON'T EDIT IT
+package "oboard/mocket/examples/responder"
+
+import(
+  "moonbitlang/core/json"
+)
+
+// Values
+
+// Errors
+
+// Types and methods
+type Person
+pub impl ToJson for Person
+pub impl @json.FromJson for Person
+
+// Type aliases
+
+// Traits
+