Add Markdown footnote support to from_markdown

[MattFisher]

Parse standard Markdown footnotes (text[^label] references and [^label]: definition lines) into Substack's footnoteAnchor inline nodes and footnote blocks.

Footnotes are numbered by order of first reference and labels may be numeric or named.

Also adds Post.footnote_anchor() and Post.footnote() helpers for building footnotes manually, plus tests.

[ma2za]

I found a regression in the footnote pass: it runs before/after Markdown parsing at document scope, so footnote-like text inside code can be removed or rewritten.

Could you add regression coverage for these cases before merge?

def test_footnote_definition_inside_fenced_code_stays_code():
    post = make_post()
    post.from_markdown("```\n[^1]: not a footnote\n```")
    content = body_content(post)
    assert len(content) == 1
    assert content[0]["type"] == "codeBlock"
    assert content[0]["content"][0]["text"] == "[^1]: not a footnote"


def test_footnote_reference_inside_fenced_code_stays_text():
    post = make_post()
    post.from_markdown("```\ncode [^1]\n```\n\n[^1]: note")
    content = body_content(post)
    assert content[0]["type"] == "codeBlock"
    assert content[0]["content"][0]["text"] == "code [^1]"


def test_footnote_reference_inside_inline_code_stays_text():
    post = make_post()
    post.from_markdown("`code [^1]`\n\n[^1]: note")
    content = body_content(post)
    assert content[0]["type"] == "paragraph"
    assert content[0]["content"][0]["text"] == "code [^1]"
    assert content[0]["content"][0]["marks"] == [{"type": "code"}]

The first test currently fails on this branch with content[0]["type"] == "footnote" instead of codeBlock. The fix should keep footnote extraction out of fenced code blocks and skip injecting anchors into inline code-marked text nodes.