fix code generate path bug.
fix code generate path bug.

There are two bugs fixed here.

1. The static constructor of LuaCodeGen.StartUp is called before
unity’s serialization works.So the value of
`SLuaSetting.Instance.UnityEngineGeneratePath` is always default.
We can move the check code to `DidReloadScripts callback` to fix it.

2.Use `Path.Combine` instead of `path+name`. If user set path =
‘slua/Unity’ without ending of ‘/’, the generated file path will become
`slua/Unity{Name}`.

Roland0511/slua,luzexi/slua-3rd-lib,jiangzhhhh/slua,jiangzhhhh/slua,luzexi/slua-3rd-lib,soulgame/slua,mr-kelly/slua,luzexi/slua-3rd-lib,soulgame/slua,jiangzhhhh/slua,jiangzhhhh/slua,luzexi/slua-3rd,Roland0511/slua,soulgame/slua,luzexi/slua-3rd,haolly/slua_source_note,luzexi/slua-3rd,mr-kelly/slua,haolly/slua_source_note,luzexi/slua-3rd-lib,shrimpz/slua,yaukeywang/slua,jiangzhhhh/slua,Roland0511/slua,pangweiwei/slua,Roland0511/slua,luzexi/slua-3rd-lib,shrimpz/slua,luzexi/slua-3rd,Roland0511/slua,yaukeywang/slua,luzexi/slua-3rd-lib,mr-kelly/slua,yaukeywang/slua,yaukeywang/slua,pangweiwei/slua,haolly/slua_source_note,pangweiwei/slua,soulgame/slua,mr-kelly/slua,luzexi/slua-3rd,soulgame/slua,jiangzhhhh/slua,haolly/slua_source_note,yaukeywang/slua,mr-kelly/slua,Roland0511/slua,soulgame/slua,haolly/slua_source_note,pangweiwei/slua,yaukeywang/slua,mr-kelly/slua,haolly/slua_source_note,luzexi/slua-3rd,pangweiwei/slua