Add drag-select to Multi dropdowns

[corradedied]

Selecting many values in a long Multi dropdown (e.g. a player list) required clicking each item individually. Holding left click and dragging over items now selects or deselects them in one gesture.

Three upvalues (DragSelecting, DragSelectState, DragInputEndedConn) and a StopDragSelect helper are declared outside BuildDropdownList so they survive list rebuilds. BuildDropdownList calls StopDragSelect at the top to cancel any in-progress drag before destroying and recreating buttons.

For each non-disabled item in a Multi dropdown:

• InputBegan (MB1) starts the drag, locks the toggle direction to the opposite of the pressed item's current state, applies it immediately, and attaches a UserInputService.InputEnded listener that fires callbacks once and clears drag state on mouse release.

• MouseEnter extends the drag to each item the cursor passes over, applying the locked direction and respecting AllowNull.

• MouseButton1Click returns early when DragSelecting is true to suppress the synthetic click Roblox fires after InputEnded, preventing the first item from being double-toggled.

Callbacks fire once at the end of the gesture rather than per item to avoid hammering Callback/Changed during a fast drag. Single-select dropdowns are unaffected.