Fix for ::: block rendering

[chkohner]

There were a couple issues with the previous implementation of ::: block rendering.

• Webpack needed an extra entry point so it wouldn't tree-shake markdown-it-container away completely
• The previous logic was not producing correct input syntax (not sure when this was broken).
• There was the potential to clobber tokens outside the ::: open/close pair due to the use of .entries(), which has been cleaned up and optimized a bit.
• The container was catching all ::: blocks, not just mermaid ones (#93)

Fixes #83 Fixes #94

[chkohner]

Any chance we can get a release with this? 😇

[mjbvz]

Thanks!