Configure default language for fenced code blocks

[varac]

Context

When reformatting indented code blocks, I'd like to have an option to set the default language added to the fenced code block that mdformat creates.
I habe a lot of old documentation with indeded code blocks, and 99% of them are sh lang, which I manually need to add after reformatting.

Proposal

Add an option to the config file like:

fenced_lang = sh

Which will automatically use

```sh 

instead of

```

when reformatting indented code blocks.

Tasks and updates

No response

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