myst formatting directive with only 1 parameter

[paugier]

Context

```{exercise-start}
:label: ex1
```

is formatted as

```{exercise-start}
---
label: ex1
---
```

Proposal

I think it is too verbose for this case, especially when we have several directives like that in a file.

When there is only 1 parameter, it should be kept with the syntax :label: ex1.

Tasks and updates

No response

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Welcome to the EBP community! 🎉

[hukkin]

Thanks for the issue!

Transferred this to mdformat-myst as mdformat core does not format myst directives.

I feel like the current style is best in line with how mdformat typically format's things: strict, simple, consistent. But I know that e.g. @chrisjsewell probably disagrees (see the discussion), and I understand their side as well.

[paugier]

Another case for which really I wish to have the shorter format

```{math}
:label: rayleigh_eq
\varphi'' - k^2 \varphi - \frac{U''}{U-c} \varphi = 0
```

instead of

```{math}
---
label: rayleigh_eq
---
\varphi'' - k^2 \varphi - \frac{U''}{U-c} \varphi = 0
```

I have files with several equations like that. Two more lines per equation make the whole file much longer and less readable.