Add hidden fillEmptyTiles function

Added a hidden function to fill empty tiles.

Author: Jaqster

docs/skillmap/3-in-a-row/3-in-a-row2.md

@@ -360,6 +360,8 @@ hint~
 let symbols: Image[] = []
 let cleared = false
 let cursor: Sprite = null
+// @hide
+function fillEmptyTiles () {}
 function clearMatches () {
     cleared = false
     for (let gem of symbols) {