Contour line display

[markschlosseratbentley]

This PR adds support to iTwin.js for contour display of specified subcategories within the iModel.

TODO (before this PR merges):

• [x] Support "just lines" display (show just lines, not the underlying geometry)

TODO (in near future):

• [x] Bug: At certain LODs, certain elements within certain tiles can display the incorrect (or undefined) contour styling
• [x] Maybe not Requirement: Apply contour lines default (Apply to all other geometry)
• [ ] Snapping
• [x] Ability to enable transparency in the underlying mesh, but not in the contour lines

Example rendering:

[mergify[bot]]

This pull request is now in conflicts. Could you fix it @markschlosseratbentley? 🙏 To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/