Add demo for JEP 334 - JVM Constants API (Java 12)

[Abhineshhh]

Description

Implements demo for JEP 334 - JVM Constants API introduced in Java 12.

Closes #317

Changes

• Created ConstantsAPIDemo.java demonstrating the java.lang.constant API
• Added JDK12Info.json with metadata for Java 12 JEPs
• Updated Java12DemoLoader.java to register the new demo

Demo Features

The demo showcases:

1. ClassDesc - Nominal descriptors for class types (primitives, references, arrays)
2. MethodTypeDesc - Method signatures and type modifications
3. MethodHandleDesc - Static and virtual method handle descriptors
4. DynamicConstantDesc - Bootstrap methods and dynamic constants

Checklist

• Follows package structure: org.javademos.java12.jep334
• Implements IDemo interface with proper info() and run() methods
• Uses Markdown-style documentation (///)
• Added JSON entry to JDK12Info.json
• Registered in Java12DemoLoader.java using JEP number as key
• Includes @author attribution
• Working code examples covering all major APIs