optimize: Simplify the export-default-from transform

[magic-akari]

Q	A
Fixed Issues?	No
Patch: Bug Fix?	No
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

See the test cases

[JLHwung]

Further optimization idea: we can merge { default as v } with the next specifier if exists.

[magic-akari]

Sure. And here come a case

export v, * as ns, { x, y as w } from "mod"; 

if there is a namespace compound, should we merge it?

// a.js
export { default as v,  x, y as w} from "mod";
export * as ns from "mod";

The export order will change.

import * as ns from "a.js";
Object.keys(ns) // order changed

[magic-akari]

Ok, it will be merged if there is no namespace export.

[JLHwung]

Thanks.