一些类型修改

[xiaoqch]

null改为undefined，参考 ScriptX 各种值类型

注意: 对于JS有null和undefined两种类型，在ScriptX中统一收归为script::Null类型。同时ScriptX的script::Null在传递给JS的时候统一为undefined。

引擎未定义枚举改为const enum，这样在编译时会被内联为枚举值

[superx101]

typescript项目编译失败:

platforms/javascript/src/EventAPI/EntityEvents.d.ts(3,14): error TS2567: Enum declarations can only merge with namespace or other enum declarations.

platforms/javascript/src/GameAPI/Basic/types.d.ts(7,20): error TS2567: Enum declarations can only merge with namespace or other enum declarations.

[xiaoqch]

typescript项目编译失败:

platforms/javascript/src/EventAPI/EntityEvents.d.ts(3,14): error TS2567: Enum declarations can only merge with namespace or other enum declarations.

platforms/javascript/src/GameAPI/Basic/types.d.ts(7,20): error TS2567: Enum declarations can only merge with namespace or other enum declarations.

有个多余的定义，修了