ftplugin: Creating a collapsed section with command

Before:

```
hello, world.
```

After:

```md
<details>
hello, world.
</details>
```

or, with `<summary>` if args given.

```md
<details>

<summary>code sample</summary>

hello, world.
</details>
```


## Appendix:

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/organizing-information-with-collapsed-sections