Closes #8124

Fix incorrect normalization of contours.value for interval constraints.

This causes cases such as:

[]           -> [NaN, NaN]
[5]          -> [5, NaN]
[2,4,8,10]   -> [8,10]

This PR normalizes them as:

[]           -> [0,1]
[5]          -> [5,6]
[2,4,8,10]   -> [2,4]

The issue is in constraint_defaults.js, where the array length is checked against contours.length instead of contours.value.length, and arrays longer than two values are handled with slice(2) instead of slice(0, 2).

The relevant logic is this part:

This affects both contour and contourcarpet, since they share the same constraint defaults.