Add collapsible toc

[giuseppe-dandrea]

Added the collapsible toc in the options for the extension.

[simov]

Thanks, note that Markdown Viewer v5 just got released, so you may want to have a look as it is a substantial update that contains a lot of new features, updates and bugfixes. I will have a look at your implementation.

[hey-tommy]

Is there anything stopping having this commit merged?

Since it adds the collapsible TOC as a separate content option switch (that is, it doesn't replace the existing TOC option, it adds an additional TOCC option right below the existing TOC option), and all the code changes are strictly additions (no lines are changed or deleted), it seems pretty low-risk to add it as an additional feature.

Thanks to @giuseppe-dandrea for the work, and obviously, thanks to @simov for the superb extension!