Multi-selection support for all applicable functionalities

[jellypuno]

Allow multi-selection of PDS members in the dataset tree. This will allow the user to perform the following:

• Copy multiple PDS members to another PDS
• Delete multiple PDS Member

This may involve UX

[katelynienaber]

Multi-delete of DS + members will be completed with this PR: #1323

[jellypuno]

MVP:

• Copy from PDS to PDS
• Process is done through context menu. PDS1: Right-click > Copy then PDS2: Right-click > Paste
• Research for Drag and Drop, if it is possible
• Questions:
  • What happens if there are duplicates? Pop a confirmation message?
  • Should we add a max number of members to copy? Research needed.

[katelynienaber]

Note: Drag and drop is not possible now, but may soon be available! It's in the "proposed" API for VSCode, but not in the currently-released stable version.

I am not sure when it would be released into the stable version, but it is currently in the "feedback-gathering" phase. It can be tested out in any project by importing the Nightly build of VSCode's API. But we can't release an extension using that version of the API, because the Microsoft team considers it unstable :)

[jellypuno]

Connected Discussion: #1130

[jellypuno]

Make this into an EPIC and break it to smaller stories:

Data set tree:
Multiple member select, for job submission?
Multiple select for copy/paste (could also multiple copy/paste member or file content as text, with some formatting to separate contents of multiple files)
USS Tree:
Multiple select for copy/paste
Jobs tree
Multiple job select for downloading spools
Multiple job select for Issue stop command
All trees:
Multiple select for Add to favorites (this could also be cross-tree, ie. from Jobs & Data Sets at the same time, or whatever)
Multiple select for Remove from favorites
Multiple select for hide/delete session

[jellypuno]

For reference, VSC issue about drag and drop on tree views: microsoft/vscode#32592

[jellypuno]

Drag and Drop is part of the Nov 2021 plan: microsoft/vscode#136630