Version constants for Lua

Pretty simple thing. Allows scripters to use VERSION, SUBVERSION, and VERSIONSTRING to check the game's version and potentially futureproof scripts. Have tested and confirmed working on my end.

src/dehacked.c

@@ -6941,6 +6941,8 @@ struct {
{"PUSHACCEL",PUSHACCEL},
{"MODID",MODID}, // I don't know, I just thought it would be cool for a wad to potentially know what mod it was loaded into.
{"CODEBASE",CODEBASE}, // or what release of SRB2 this is.
+	{"VERSION",VERSION}, // Grab the game's version!
+	{"SUBVERSION",SUBVERSION}, // more precise version number
// Special linedef executor tag numbers!
{"LE_PINCHPHASE",LE_PINCHPHASE}, // A boss entered pinch phase (and, in most cases, is preparing their pinch phase attack!)
@@ -8213,6 +8215,9 @@ static inline int lib_getenum(lua_State *L)
} else if (fastcmp(word,"gravity")) {
lua_pushinteger(L, gravity);
return 1;
+	} else if (fastcmp(word,"VERSIONSTRING")) {
+		lua_pushstring(L, VERSIONSTRING);
+		return 1;
}
return 0;