Have virtualSize default to windowSize when not specified

I'm new to Korge, and I found a really confusing feature yesterday. I had a 1920x1080 screen (that I specified with `windowSize`), but when I drew a 1000x700 rectangle it almost covered the whole screen. I later realized that I also had to specify a `virtualSize`. I propose that, when `virtualSize` is not specified within the `Korge()` function, the `Korge.Config` dataclass, or the `Module`, it should default to the `windowSize`.

Locate the Korge() entry point, Korge.Config dataclass, and Module definitions mentioned in the issue, then trace how windowSize and an omitted virtualSize are handled. Confirm the default is consistent across all three paths and add regression coverage showing that virtualSize inherits windowSize when unspecified.

Written by the indexing model from the issue text.