libghostty: expose viewport active state

Expose whether the terminal viewport is currently pinned to the active
area through the libghostty-vt terminal data API. Previously embedders
could only infer this from scrollbar geometry, which was indirect and
could require the more expensive scrollbar calculation.

The new GHOSTTY_TERMINAL_DATA_VIEWPORT_ACTIVE value returns the exact
PageList viewport state as a bool. The scroll viewport test now verifies
the value while moving between the active area and scrollback.