Markdown: add support for "Raw" or "Inline" HTML

[fingolfin]

In doc/src/manual/noteworthy-differences.md:440 there is a Markdown table containing HTML, namely <br>. It renders as text, not as a line break. E.g. we see

assembling<br>software modules

instead of

assembling
software modules

in one of the table cells

With this PR, we parse inline / raw HTML correctly, which is the basis for fixing the above.

However, to actually work, also Documenter and/or MarkdownAST will have to learn about the new Markdown.HTMLInline node type. (And I am guessing also about the other node types I recently added: Markdown.HTMLBlock and Markdown.Strikethrough). Otherwise, the example table cell from above renders as

assemblingMarkdown.HTMLInline("<br>")software modules

which I'd say is even worse than the current status quo.

Therefore I've marked this PR as draft: before merging this, I'd like to have patches for Documenter and MarkdownAST ready.

CC @mortenpi

[fingolfin]

Oh yeah: if anyone is able to help with this, that'd be appreciated.