Add include attribute for code fences

[cestef]

IMPORTANT: Please do not create a Pull Request adding a new feature without discussing it first.

https://zola.discourse.group/t/include-and-copy-attributes-for-code-fences/

The place to discuss new features is the forum: https://zola.discourse.group/
If you want to add a new feature, please open a thread there first in the feature requests section.

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

(Delete or ignore this section for documentation changes)

• Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• Have you created/updated the relevant documentation page(s)?

This PR adds support for two fence settings:

```include=

• Allows including a local file as the content of this codeblock, useful for large files

```copy

• Adds a data-copy attribute to the code-block, useful if you want to add copy buttons only to some blocks.

[Keats]

Can you cherry pick the commits on the new next branch?

[cestef]

@Keats this should do it

[Keats]

Can you split the include and copy feature in different PRs? The copy one is easy but the include is a bit more involved (error handling, maybe caching etc)

[kKdH]

Hey guys, I created a post in the forum and asked for a similar feature to this one. Maybe you are interested to take a look.