Add Shift+Enter to finish polygon creation by closing the shape

[PrateekSingh070]

While creating a polygon or extending a polyline, pressing Shift+Enter now closes the shape as a polygon. This requires at least three points, matching the existing behavior when clicking the first point to close. Plain Enter still finishes as a polyline.

Implements the TODO left in createpolygonobjecttool.cpp and follows up on the Backspace support added in #4372.

[Copilot]

Pull request overview

Adds a keyboard shortcut to close an in-progress polygon as a polygon (instead of finishing as a polyline), and documents the new behavior.

Changes:

• Handle Shift+Enter in CreatePolygonObjectTool::keyPressed to finish as a polygon when there are at least 3 points.
• Update user-facing docs and release notes to mention the new shortcut.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/tiled/createpolygonobjecttool.cpp	Implements Shift+Enter handling to close the shape and finish as a polygon.
NEWS.md	Notes the new Shift+Enter behavior in Unreleased changes.
docs/manual/objects.rst	Documents Shift+Enter as an alternative to clicking the first point to close a polygon.
docs/manual/keyboard-shortcuts.rst	Lists Shift+Enter as “finish as polygon” while creating polygons/polylines.