[Feature Request] select add fixed width option

### Existing Component
是

### Component Name
select

### Description
select add fix-width option
default is false which same as before
when set true, dropdown width is alaway equal to parent select

select选择框增加一个固定宽度的属性
默认值是false，跟以前一样
如果设置为true，下拉框的宽度固定，总是跟select一样

before
after