Expose P_CheckSkyHit to Lua

Part of a series of isolations from !1472 (closed). This one exposes P_CheckSkyHit to Lua. Checking if an object touched the sky (ie a thok barrier) can often be obnoxious to write the logic for, so this merge request eases the process by exposing a function that the source uses for a similar purpose. It has been modified to have a line_t parameter instead of being hardcoded to always check ceilingline.

In addition to all this, I have made an addition change to the function. The function now also considers horizon line linedefs to be sky, as they are often used as such. This is what line->special == 41 means in the code here.