Add intersects XPath function

[lognaturel]

Description

Following the spec at https://forum.getodk.org/t/odk-xforms-spec-proposal-add-intersects-xpath-function-to-detect-geotrace-shape-intersections/57121

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Checklist

• Does it need UI/UX design?
  • Yes. Describe the UI/UX requirements or attach a Figma link in the "Design" section.
  • No
• Does it need API design?
  • Yes. Specify the endpoints, methods, or data structures needed.
  • No
• Does it need design around state flow?
  • Yes. Outline the state flow requirements.
  • No
• Does it need a test plan?
  • Yes. Add the test plan as a comment in this task or attach the document.
  • No

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User Stories

As a form designer
I want to write expressions based on whether segments in a line or polygon intersect
So that I can warn or block end users who try to capture self-intersecting geometries

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Additional Notes

This is not a Web Forms priority but wanted it track so we don't forget it! Once the work is complete in Collect it should be a fairly straightforward port.

[latin-panda]

Reminder that we need to reevaluate whether the first and last points of a closing shape are considered the same (original comment)