Allow extending multi-selection with Shift while preventing duplicate…

[Shyam-123pandey]

This change improves multi-selection usability by allowing users to extend the
current selection using Shift + click and Shift + lasso.
Vedio of fixes: https://drive.google.com/file/d/13wMn-91atIUZN0aL4xTrUWcIgXCHHrDE/view?usp=sharing

Previously, new selections would replace the existing selection, making it
difficult to incrementally build a multi-selection.

The implementation merges the current selection (context.selectedIDs())
with the newly selected IDs while ensuring uniqueness using Set() to avoid
duplicate entity IDs.

Behavior after this change:

• Click → replaces selection
• Shift + click → adds entity to selection
• Shift + lasso → adds entities to selection
• Selecting an already selected entity does not create duplicates.

This addresses usability issue #11134.

[Copilot]

Pull request overview

This PR improves lasso multi-selection in iD editor by merging newly lasso-selected entities with the existing selection when Shift is held, rather than replacing the selection. It addresses issue #11134 which requested the ability to incrementally build multi-selections.

Changes:

• Modified behaviorLasso's pointerup handler to accept the event parameter and merge current selection with new lasso selection when Shift is held, using Set for deduplication.
• Removed a blank line in modules/modes/select.js (cosmetic).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
modules/behavior/lasso.js	Added Shift-aware selection merging in lasso pointerup handler
modules/modes/select.js	Removed a blank line (cosmetic only)

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[Shyam-123pandey]

Hello Sir @tyrasd please check PR and i have one doubt in reply section, please tell there.