Always prepend directive names with `@`

[ddebrunner]

Describe what is missing

In GraphQL directives are always used with @, even when defined, e.g. @include.

When magidoc documents them it displays them without the at-sign, to me it's jarring and means that when I look at the list it's not immediately obvious the context is a directive.

So adding @ signs like this (not in red though!)

[pelletier197]

Hi @ddebrunner sorry for the late reply. This is a great suggestion. Would you be interested in contributing and try to implement it in a PR?

I would mind a little color for the @ if you want, otherwise it might look bland if it's just all black, but I leave it up to you to decide 🙂