Support specialized role

The specialized role feature of ReST allows to input inline raw code. Supporting this feature would allow among other for punctual use of latex formating commands that have no equivalent in markdown, nor ReST.

It relies on the raw directive which is functionning well with MyST (as any other “normal” directive). However, adding (as suggested in the documentation linked above) the following somewhere (say in rst_prolog):

.. role:: raw-latex(raw)
   :format: latex

does not produce the expected result. The code latex (or html or whatever) raw code is not interpreted. Instead something like :raw-latex:‘RAW_CODE_TO_BE_INTERPRETED‘ is typeset in way as to indicate a problem (it depends on the output processor).

[chrisjsewell]

This should be closed in dcbd5ef 😄 , can you give it a go?

Note, the role definition should also be written in MyST, i.e

```{role} raw-latex(raw)
:format: latex
```

{raw-latex}`\tag{content}`

[tavin]

This is a handy trick. Looking also at docutil_directives.md I would guess that I should be able to do something like this as well:

```{raw} raw
:format: latex

\tag{content}
```

Why doesn’t it work? Is it mandatory to define a role?