Add scopes for directives

[Qrokqt]

I'd like to color vue directives differently than normal attributes, but there aren't scopes to support it.

At the moment in sublime, v-on:mousedown or @mousedown will have the scopes:

text.html.vue
meta.tag.block.any.html
entity.other.attribute-name.html

while in vscode has the scopes:

source.vue
text.html.vue-html
meta.tag.any.html
meta.directive.vue
entity.other.attribute-name.html

Could we get a meta.directive.vue scope equivalent in sublime?

[FichteFoll]

With #216, directives have an embedded scope indicating that they use javascript for interpolation.