Include VG in Toplevel and exercices

[maiste]

Hi,
this is a PR to partially fix #237.
Content of the PR:

• It allows using Vg (link) in the toplevel.
• It displays Vg.image in the toplevel` with pretty_printing.
• It imports a new function to print svg string

 val print_svg: string -> unit

• It allows using libraries such as VG in the exercise section.

[maiste]

Hi,
I've rebased and refactor a bit this old PR. It allows displaying vg image in the toplevel. It needs to be test on another machine: it works on mine.

To be clear, this does not support grading image, only displaying them.

@yurug what's your point of view? Should we merge only the displaying and create another PR for the grading part?

[maiste]

However, when I try to open an exercise I've got this error:

It's strange, I don't have this issue. It works on the PopOs version of:

• Chromium 83.0.4103.116
• Firefox 89.0.1

[hferee]

I just tried it as well. The image is properly displayed in the toplevel and the exercise still works.
Thanks for the pull request by the way.

[maiste]

OK, @EmileRolley I don't know where it can come from...

@hferee thanks! It's an old PR I've opened, but now it has been refactored, it might be mature for merging (as long as we ensure it works).

[yurug]

@maiste I have made minor comments.

[EmileRolley]

OK, @EmileRolley I don't know where it can come from...

Okey, after some opam install it finally works!

[yurug]

@EmileRolley @maiste Anything more? Can I merge?

[EmileRolley]

@EmileRolley @maiste Anything more? Can I merge?

It's ok for me.

[maiste]

The CI seems to stuck on Mac, whereas it wasn't the case before I apply the path. Is it possible to manually run it again?
Edit: It seems good to me now.

@yurug, good for merging to me.

[yurug]

Thanks!