Add libavoid router support

[Geliogabalus]

Summary

• Adds a new @joint/routers-avoid package integrating the libavoid routing library (main-thread and Web Worker providers) as a JointJS router.
• Adds an avoid-router example app demonstrating the router on a large sample graph.
• Minor fixes in rightAngle router and LinkView needed to support the new router.

Test plan

• yarn test passes for joint-routers-avoid package
• avoid-router example builds and runs (yarn ensure-examples-can-build)
• Manually verify routing behaves correctly with both main-thread and worker providers

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