Parse roles in directive front matter keys

[vepain]

Context

Let suppose I wrote this in RST:

.. data:: IdT

    Identifier type.

    :value: :class:`~typing.Hashable`

I would like to write this using MyST-parser:

```{data} IdT
---
value: {class}`~typing.Hashable`
---
Identifier type.
```

But at doc compile time, it raises the following error:

Directive 'data': Invalid options YAML: while scanning for the next token found character '`' that cannot start any token

Proposal

Each value of each YAML front matter key should be parsed by MyST-parser. I think the error comes from YAML parsing.

One idea is to add string delimiter, and then parse the string:

```{data} IdT
---
value: "{class}`~typing.Hashable`"
---
Identifier type.
```

Tasks and updates

• [ ] Parse the strings given as key's value in YAML front matter of directives
• [ ] Add to the doc the specific case of inserting a role in YAML keys, and say that the role must be delimited by string delimiters

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