Add `default-floating-position relative-to="cursor"` window rule

[YaLTeR]

See: https://github.com/YaLTeR/niri/discussions/972

If the cursor (or rather, the window) is on a different output or workspace, then just ignore the window rule.

[Axlefublr]

:o this would be amazing for something like kruler (niche example but still hahaha)