initialPosition should set both coordinates when one number exists

[cinnamon-msft]

Environment

Windows build number: 10.0.19041.0
Windows Terminal version (if applicable): 0.8.10091.0

Any other software?

Steps to reproduce

Set initialPosition to "#" (any number will work).

Expected behavior

That number will set both the X and Y coordinates of the Terminal on launch. This matches the functionality of padding.

Actual behavior

Only the X coordinate gets set.

[DHowett-MSFT]

This'll be in contravention to the agreed-upon spec for launch position. We need consensus before changing it.