add AST for the module attribute proposal (#11015)
add AST for the module attribute proposal (#11015)

Introduce a `ImportAttribute` structure and add an `attributes` key on
the `ImportDeclaration` node.

[skip ci]
babel/babel,jridgewell/babel,babel/babel,jridgewell/babel,hzoo/babel,jridgewell/babel,hzoo/babel,hzoo/babel,jridgewell/babel,kaicataldo/babel,babel/babel,kaicataldo/babel,kaicataldo/babel,babel/babel,kaicataldo/babel,hzoo/babel