JEP 378- Text Blocks enhancements added

[SanjanaMahapatra]

This is the enhancement added for the JEP 378- Text Block which was assigned.

Implemented demo for JEP 378 - Text Blocks (Java 15), along with linking classes for the preview versions in Java 14 (JEP 368) and Java 13 (JEP 355).

Java 15 - JEP 378: Created TextBlockDemo.java and added some more examples showcasing the use case of text blocks. And removed the file from the java17 folder.

Linking Classes List -

1. Java 14 - JEP 368: Created TextBlockSecondPreviewDemo.java (second preview) - Added the relevant examples.
2. Java 13 - JEP 355: Created TextBlock.java (first preview)

Resources Folder -

1. Updated JDK15Info.json, adding - JEP 378
2. UpdatedJDK14Info.json, adding - JEP 368
3. Updated JDK13Info.json, adding - JEP 355
   Registered all demos in respective Java13DemoLoader, Java14DemoLoader, Java15DemoLoader classes
   Updated the JEPInfo.java and updated the list of sources to add JDK 13 to their list.

[AloisSeckar]

Hi @SanjanaMahapatra and thank you for your contribution. I see two things to change and one to check and maybe change. Please, have a look.

[SanjanaMahapatra]

Hi @AloisSeckar , please find the commit id for the changes which you have requested.
3583427.
Also, there were merge conflicts which was coming few minutes back in Java14LoaderDemo.java file with the addition if NullPointerDemo.java class, have resolved the merged conflicts and added the required changes.
Thanks.