Shortcut to open the next/previous file in package tree

### Description

It would be helpful to have a shortcut to open the next file in the package tree.
The same function should also be available for the previous file in the package tree.

Example:
- ezvcard.property
- Address.java
- Agent.java (selected)
- Anniversary.java

STRG+SHIFT + UP = Jumps to Address.java (Open previous file)
STRG+SHIFT + DOWN = Jumps to Anniversary.java (Open next file)

### Use case/motivation

Work more efficiently.

### Related issues

_No response_

### Are you willing to submit a pull request?

No

Start by locating the NetBeans package tree navigation and its existing keyboard shortcut or action handling, using the example with Agent.java selected as the behavior to trace. Add previous-file and next-file navigation for the package tree, then verify that Ctrl+Shift+Up and Ctrl+Shift+Down open the adjacent files described in the issue.

Written by the indexing model from the issue text.