feat(compiler-sfc): transform srcset

[underfin]

No description provided.

[yyx990803]

Could you update this one based on #500?

[underfin]

Year. I will work it.

[underfin]

<img src="./logo.png" srcset="./logo.png 2x"/>
<img src="./logo.png" srcset="./logo.png 2x"/>

Like this. The hoisted expression is const _hoisted_1 = _imports_0 + '2x'.This should can be reuse with hoist
same style and the same url with same fragment.The optimize is not implement. Should add this to TransformContext, like this blow, imports add like this as well.

export interface TransformContext extends Required<TransformOptions> {  
  hoist(exp: JSChildNode): SimpleExpressionNode
}

hoist(exp) {
     if(// same exp){
        // return existing exp
     }
      context.hoists.push(exp)
      return createSimpleExpression(
        `_hoisted_${context.hoists.length}`,
        false,
        exp.loc,
        true
      )
    },

[vue-bot]

Hey @underfin, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚