Add line segment-line segment intersection

[shehzan10]

Find the intersection between 2 line segments in 2D.

[cesium-concierge]

Thanks for the pull request @shehzan10!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[hpinkos]

@shehzan10 merge in master. Other than that one comment, this looks good!

[shehzan10]

@hpinkos Updated.

[hpinkos]

Thanks @shehzan10 !