Expand DomConverter.blockElements with missing elements

[jacekbogdanski]

Suggested merge commit message (convention)

Fix (engine): DomConverter.blockElements is missing some of the HTML block element names. Closes #9801. Closes #7863.

[maxbarnas]

All good, restarted the build job. Will wait for it to finish before merging.

[Reinmar]

Other (engine): Changed DomConverter.blockElements type to Set.<String>. Closes #3920.

This sounds like a breaking change. This is a part of a public API and we recommended adding elements to that array.

Secondly, the ticket that I opened in 2016 was opened when this thing was added in the first place and it made more sense then. Changing this now, without a good reason has a dubious ROI in my opinion. The cost here is in the fact that it's a breaking change.

[jacekbogdanski]

@Reinmar thanks, I've reverted the type change. I've also changed the merge message into a fix, as it seems to be fixing some cases in code-block and markdown-gfm plugins.