[Feature Request]: User Defined Java Class - Code editor improvements

### What would you like to happen?

I would like to suggest some improvements for the code editor in User Defined Java Class:

- **Syntax highlighting**: it's quite hard to tell even what is actual code and what is a comment in a mono-color text...
- **Memory of indentation**: currently when pressing ENTER the cursor goes back to the beginning of the line, and if you indented the code to keep it ordered you need to retype all the spaces or tabs at every line

I also noticed that many code snippets (by the way, it's **snippets** not _snippits_ 😄) are still wrong... (see bug #2718)

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui, Component: Transforms

Start by locating the Hop GUI User Defined Java Class code editor and review bug #2718 for the related snippet problems. Done means the editor provides syntax highlighting, preserves indentation after pressing ENTER, and the identified snippets are corrected.

Written by the indexing model from the issue text.