Nested documentation strings

[yloiseau]

It is not currently possible to use documentation strings in code samples in documentation strings...

for instance:

----
This is the documentation of the `foo` function

Usage example:

    ```
    ----
    This is a sample main function
    ----
    function main = {
        foo()
    }
    ```

In this example, we call `foo` in a `main` function
----
function foo = -> null

The golo parser makes an error on "This".

[jponge]

Inception 🤣