Constraining the entire end should be equivalent to constraining both endX and endY

[adamchalmers]

Currently the frontend checks if a path segment is constrained by checking that:

• The end point is an array of length 2
• Both X and Y components of that array are constrained

e.g. line(endAbsolute = [myX(), myY()] is fully constrained.

But this fails to recognize some kinds of fully-constrained lines. For example it wouldn't recognize that line(endAbsolute = myXY()) is fully constrained. Even though it's basically equivalent -- it's assigning the endpoint to a function's output.

This unblocks #5096