Sketch2: add line

[bencodeorg]

Implements a line as an edge that connects two "ghost nodes".

As an alternative, I went down the path of implementing this as a "custom node" (very rough): https://github.com/code-dot-org/code-dot-org/compare/ben/sketch2-line

I think implementing it as an edge makes sense longer term because we'll be able tap into the built-in goodness that React Flow includes for "lines" as part of its edge implementation.

Their docs (linked below) also mention the idea ("What about an “edge” not connected to any nodes at all?") of an edge with no nodes (ie, what we're trying to do), so that seems like a reasonable standard approach.

Links

• "Temporary Edge" docs: https://reactflow.dev/examples/edges/temporary-edges
• Jira: https://codedotorg.atlassian.net/browse/AFL-576

Testing story

Tested manually (see video above).

Follow-up work

• Center drag handles on edges of the line. This was trickier than I expected and I wanted to get a V0 out, so punted for now.
• Add support for arrow heads, colors, thicknesses, etc.
• Would be nice to have a drag target on the center of the edge, although that might be tricky.