[Feature Request] Multi select in tree view

It would be good to be able to move/duplicate/delete/etc. multiple request and folders in the tree view at once using combinations such as:

**Mouse**
shift+`left click`: Select all items from the currently focused item to the clicked item
ctrl+`left click`: Add the clicked item to the selection

**Keyboard**
shift+↑: Add the item above the focused item to the selection (or reduce the selection)
shift+↓: Add the item below the focused item to the selection (or reduce the selection)
ctrl+↑: Move the focus up without selecting the item
ctrl+↓: Move the focus down without selecting the item
ctrl+space: Add/remove the focused item from the selection

No files, tests, or entry points are named in the issue. Start by locating the tree view and its existing focus, selection, and move/duplicate/delete handling; reproduce the requested mouse and keyboard interactions there. Done means multiple requests and folders can be selected and those operations work on the selection without breaking keyboard navigation.

Written by the indexing model from the issue text.