Add demo for JEP 371 - Hidden Classes (Java 15)

[Abhineshhh]

Description

Implements demo for JEP 371 - Hidden Classes introduced in Java 15.

Fixes #288

Changes Made

• Added HiddenClassesDemo.java with complete implementation using ASM library
• Uses ASM to generate bytecode for a simple class at runtime
• Demonstrates all key features of hidden classes:
  • Creation via Lookup.defineHiddenClass()
  • Non-discoverability with Class.forName()
  • Nest membership
  • Method invocation on hidden class instances
• Added ASM 9.7.1 dependency to pom.xml
• Added JEP 371 entry to JDK15Info.json
• Registered demo in Java15.java initializer

Checklist

• Follows contribution guide
• Proper Markdown-style documentation
• Calls info(371) as first statement
• JSON entry added in correct order
• Demo registered in initializer
• No unrelated changes

[Abhineshhh]

@AloisSeckar have a look to the committed changes

[AloisSeckar]

This looks really good. I appreciate the effort. Just bumped the newer version of the ASM library.