[add] show rule on raw for examples

[Mc-Zen]

This adds a show rule for raw elements with the languages

• example (for Typst markup)
• examplec (for Typst code mode)
  that will transform them into the code displayed side-by-side with the rendered output.

This addresses the third point in #32 .

Usage in documentation

In doc-comments, these languages are by default available. Note that the preamble is only applied to the markup version example and not the code version examplec since there is no good way to automatically convert the preamble between the two.

Hidden code

It is possible to add hidden code that will be executed but not shown on lines starting with >>>. This is useful for import statements, wrapping everything inside a container of a fixed size and other things.

>>> #let read(filename) = "Content of a file we cannot access. " // This line is executed but not shown 
#read("my-file.txt")

This is inspired by the Typst documentation.

Standalone usage

This show rule can be used standalone via the function render-examples().

#import "@preview/tidy:0.4.0": render-examples
#show: render-examples

```example
#rect()
```

```examplec
rect()
```

Scope

It also features a scope argument, that can be pre-set

#show: render-examples.with(scope: (answer: 42))

```example
#answer
```

Customization

The output format of the example can be customized through the parameter layout of render-examples. This parameter takes a function with two positional arguments: the raw element and the preview.

#show: render-examples.with(layout: (code, preview) => grid(code, preview))

Breaking change

In #15, this standalone example rendering was suggested. Before this PR, it was possible to write

#show raw: show-example.show-example

```typ
Hello world
```

This now breaks and should be replaced with the versions above.