Implement initial infrastructure for the Spline tool to join itself with other splines and paths

[Sidharth-Singh10]

Closes #2184 (part of part 4)

[Keavon]

!build

[github-actions]

📦 Build Complete for 3975f74
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[Keavon]

Seems like this breaks if I try to connect a spline to a line (drawn with the Line tool). There are two modes for failure:

1. Start with a line, then draw the spline and end with connecting to an endpoint of the line. They don't connect together.
2. Start with a line and a spline, then use the Spline tool to extend the spline into the line. The line disappears to the top left of the artboard with a straight line connector going to that origin corner.

[Keavon]

!build

[github-actions]

📦 Build Complete for f9fdc28
https://021d0642.graphite.pages.dev

[Keavon]

Merging with known issue in my case #2 described above being more broken than before (invalidating the graph).