Supports for Vue.js 2.6 "Macross"

[mysticatea]

https://medium.com/the-vue-point/vue-2-6-released-66aa6c8e785e

There are two new syntaxes. This parser should support those.

• v-scope directive: RFC0001 RFC0002
• dynamic directive arguments: RFC0003

[mysticatea]

Hmm.

It needs a breaking change about the tokens of VDirectiveKey nodes. Currently, because an attribute name is an atom, a VDirectiveKey node is expressed by one token. However, dynamic arguments can have a JavaScript expression, it will be expressed by multiple tokens. Though, we cannot insert any whitespace into dynamic arguments.

[mysticatea]

And we need to support .prop shorthand notation: vuejs/vue#7582

I'm worried how we express it in AST. 🤔

[mysticatea]

Added in 0f24127...3c356d9