overlapping polygons on results fixes #11

[Dexus]

This pull request enhances the part placement algorithm in main/background.js by adding a more rigorous overlap check and final verification before accepting a position. It also introduces several utility functions to support these enhancements. Additionally, a new configuration parameter is added in main/deepnest.js.

Enhancements to part placement algorithm:

• Added a theoretical placement and overlap check before considering a position in placeParts function.
• Added final verification to ensure no overlap before accepting a position in placeParts function.

Utility functions for overlap checking:

• Introduced checkPlacementOverlap function to check for overlaps with a safety margin.
• Added expandPolygon, isPointInPolygon, and doLinesIntersect helper functions to support the overlap check.

Configuration updates:

• Added overlapTolerance parameter to the configuration in main/deepnest.js.