Apply formatting to markdown code blocks (#22470)
Adds initial support for formatting Python code blocks inside Markdown
files.
- Adds `Markdown` source types/kinds
- Maps `.md` file extension to `Markdown` by default
- Uses simple regex adapted from blacken-docs to find and format fenced
python code blocks
- Dedents contents before formatting, and reapplies indent from fenced
<code>```py</code> header
- Selects `Python` vs `Stub` options based on language label on code
block
- Silently skips formatting for any code block with syntax errors or
that produce formatting errors.
- CLI tests formatting via both stdin and from filesystem
- Requires running with `--preview`, and otherwise emits formatting
error when given a markdown file
- Requires a user to `extend-include = ["**/*.md"]` if they want to
format markdown files by default
Limitations:
- Returns a formatting error if run with a range of any sort
- Ignores implicit code blocks (no code fence)
- Doesn't yet support `~~~` fences, arbitrary fence lengths, or code
blocks inside blockquotes
Issue #3792
