Use .effekt.md for literate Effekt code

[marzipankaiser]

This mirrors effekt-lang/effekt-vscode#14, i.e.:

• Use .effekt.md instead of plain .md for literate Effekt code.
• Also changes the treatment of markdown files such that code fences that are explicitly labelled with something other than
  effekt (plus optional spaces) are ignored.
  I.e. valid (opening and closing) fence delimiters:

  ```
  ```effekt
  ``` effekt
  ```effekt :mymodifier
  ```effekt:mymodifier
  ```effekt timeout=12
  

  Ignored delimiters:

  ```tex
  ```myfancyotherlanguage
  ```effektcode
  ```eff
  

[jiribenes]

LGTM

[marzipankaiser]

Note: In some of the case studies, we use something like

```
// output
```

To give the output of a function etc.
Using this PR, we could instead do:

```text
output
```

directly.

[b-studios]

Does this regex support all the crazy flags passed on the website?

https://github.com/effekt-lang/effekt-website/blob/26493b1a1063dea79c33df9f3e45be789118c37c/docs/index.md?plain=1#L144-L146

https://github.com/effekt-lang/effekt-website/blob/26493b1a1063dea79c33df9f3e45be789118c37c/docs/concepts/effect-safety.md?plain=1#L47-L57

[marzipankaiser]

It should. It does support the ones here:

effekt/examples/casestudies/ad.md

Lines 16 to 23 in c4febaa

	```effekt:sketch:hide
	module examples/casestudies/ad
	```
	```effekt:prelude:hide
	import immutable/list
	import mutable/heap
	```

(which were the motivation behind allowing (\s|:)[^`]*)? there)

So I would be inclined to say yes, though I did not specifically test for them.