Add joining of path endpoints with Ctrl+J in the Path tool

[Sidharth-Singh10]

fixes: https://discord.com/channels/731730685944922173/881073965047636018/1289168449117097994

Implements a keyboard shortcut (Ctrl+J) to close open paths, with support for multiple scenarios:

1. With selected points: Connects two selected endpoints
2. Without selection: Automatically closes single continuous subpaths by connecting their endpoints

The implementation:

• Checks that selected points are endpoints before connecting
• Handles points in different layers by copying to topmost layer
• Only connects valid endpoints (points connected to single segment)
• Creates straight line segments without handles

[Keavon]

Please see the Pen tool for the behavior and code paths that occur when the endpoints of two layers are clicked on to draw a segment between the two. You'll notice how the two layers get joined into one. Check the graph to see the before and after of the node setup. Try reusing that code for when joining points across layers, which currently doesn't work (it just draws a line that happens to go to the location of the other one, on one of the pair of layers).

That'll be for a followup PR. I'm merging this now since that's somewhat involved and the PR works well in all the other cases at the moment. Thanks.

[Sidharth-Singh10]

Got it! I’ll handle this in a follow-up PR

[Keavon]

This also relates to some of the same concepts in #2184, particularly the video mentioned in part 4 at the very end of the issue.

[Sidharth-Singh10]

Will look into it!!