重构模式环境的dropPiece和task事件为更强大的自定义和未来联网任意模式做准备

整理出所有模式的这两个事件独立到一个文件夹并整理所有模式引用它们的方式
警告：很可能有错误，需要他人帮助测试

parts/modes/survivor_u.lua

@@ -5,27 +5,7 @@ return{
         fall=10,
         freshLimit=15,
         pushSpeed=2,
-        task=function(P)
-            while true do
-                YIELD()
-                if P.control then
-                    local D=P.modeData
-                    D.timer=D.timer+1
-                    if D.timer>=math.max(300,600-10*D.wave)and P.atkBufferSum<20 then
-                        local t=math.max(300,480-12*D.wave)
-                        table.insert(P.atkBuffer,{line=generateLine(P.holeRND:random(10)),amount=4,countdown=t,cd0=t,time=0,sent=false,lv=2})
-                        table.insert(P.atkBuffer,{line=generateLine(P.holeRND:random(10)),amount=4,countdown=t,cd0=t,time=0,sent=false,lv=3})
-                        table.insert(P.atkBuffer,{line=generateLine(P.holeRND:random(10)),amount=6,countdown=1.2*t,cd0=1.2*t,time=0,sent=false,lv=4})
-                        table.insert(P.atkBuffer,{line=generateLine(P.holeRND:random(10)),amount=6,countdown=1.5*t,cd0=1.5*t,time=0,sent=false,lv=5})
-                        P.atkBufferSum=P.atkBufferSum+20
-                        P.stat.recv=P.stat.recv+20
-                        if D.wave==31 then P:_showText(text.maxspeed,0,-140,100,'appear',.6)end
-                        D.timer=0
-                        D.wave=D.wave+1
-                    end
-                end
-            end
-        end,
+        eventSet='survivor_u',
         bg='welcome',bgm='here',
     },
     mesDisp=function(P)