Update ShapeObstacleCalculator.ts to import Node

[mcwoodleAmzn]

When building a library which uses @msagl/core I get the following error:

../../node_modules/@msagl/core/dist/routing/ShapeObstacleCalculator.d.ts:10:46 - error TS2304: Cannot find name 'Node'.

10     loosePolylinesToNodes: Map<Polyline, Set<Node>>;
                                                ~~~~

Found 1 error in ../../node_modules/@msagl/core/dist/routing/ShapeObstacleCalculator.d.ts:10

There's a reference to Node in ShapeObstacleCalculator that isn't imported in the actual file. Additionally, without this import, VSCode assumes Node is the one from the TS DOM.

[levnach]

Thanks @mcwoodleAmzn !