Escape block markers on paragraph continuation lines with the GFM extension too - #316

#310 escapes a block marker at the start of a paragraph continuation line in MarkdownRenderer.render_paragraph(). But the GFM extension overrides render_paragraph() for the Markdown renderer, to write the checkbox of a task list item, and it renders the children itself, so with extensions=["gfm"] the escaping doesn't happen:

>>> md = marko.Markdown(renderer=MarkdownRenderer, extensions=["gfm"])
>>> md("foo\n    # bar\n")
'foo\n# bar\n'

<!-- before: one paragraph -->
<p>foo
# bar</p>
<!-- after re-rendering: a paragraph and a heading -->
<p>foo</p>
<h1>bar</h1>

The rendering of the paragraph's children, with the escaping, is moved to MarkdownRenderer._render_paragraph_children(), which render_paragraph() and the GFM override both use. The output of MarkdownRenderer itself is unchanged, and the GFM override still writes the checkbox.

Added test_render_paragraph_continuation_block_start to TestGFMTableLlistElement, with a task list item; its three cases fail on master. Across the GFM spec examples, the number whose re-rendered Markdown gives different HTML with the GFM extension goes from 23 to 18 (examples 19, 40, 57, 214 and 290, the same ones #310 fixed without the extension). 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