Keep the container prefix on every line of an HTML block when re-rendering Markdown - #303

MarkdownRenderer puts the quote marker or list indentation only on the first line of an HTML block. The other lines fall out of the container, so the re-rendered Markdown gives different HTML:

>>> md = marko.Markdown(renderer=MarkdownRenderer)
>>> md("> <details>\n> <summary>More</summary>\n> Hidden text\n> </details>\n")
'> <details>\n<summary>More</summary>\nHidden text\n</details>\n'

Here only <details> stays in the block quote. The same happens to a <div> or <table> in a list item and to a multi-line comment in a quote.

render_html_block() now prefixes every line like render_code_block() does, with an empty line getting the prefix without trailing spaces, as in #302. It also drops the block's trailing newline before splitting, like render_code_block(), so the block no longer ends with an extra blank line.

Added test_markdown_renderer_html_block_in_container with five cases; they fail on master. Across the CommonMark spec examples, the number whose re-rendered Markdown gives different HTML goes from 29 to 27. pytest tests, black, isort and flake8 pass, and mypy reports the same 5 errors as on master.

Used Claude (Opus) to help find and patch this; I reviewed and tested the change