format doctests in docstrings (#8811)

## Summary

This PR adds opt-in support for formatting doctests in docstrings. This
reflects initial support and it is intended to add support for Markdown
and reStructuredText Python code blocks in the future. But I believe
this PR lays the groundwork, and future additions for Markdown and reST
should be less costly to add.

It's strongly recommended to review this PR commit-by-commit. The last
few commits in particular implement the bulk of the work here and
represent the denser portions.

Some things worth mentioning:

* The formatter is itself not perfect, and it is possible for it to
produce invalid Python code. Because of this, reformatted code snippets
are checked for Python validity. If they aren't valid, then we
(unfortunately silently) bail on formatting that code snippet.
* There are a couple places where it would be nice to at least warn the
user that doctest formatting failed, but it wasn't clear to me what the
best way to do that is.
* I haven't yet run this in anger on a real world code base. I think
that should happen before merging.

Closes #7146 

## Test Plan

* [x] Pass the local test suite.
* [x] Scrutinize ecosystem changes.
* [x] Run this formatter on extant code and scrutinize the results.
(e.g., CPython, numpy.)

crates/ruff_python_formatter/src/context.rs

@@ -1,5 +1,5 @@
 use crate::comments::Comments;
-use crate::PyFormatOptions;
+use crate::{PyFormatOptions, QuoteStyle};
 use ruff_formatter::{Buffer, FormatContext, GroupId, SourceCode};
 use ruff_source_file::Locator;
 use std::fmt::{Debug, Formatter};
@@ -11,6 +11,15 @@ pub struct PyFormatContext<'a> {
     contents: &'a str,
     comments: Comments<'a>,
     node_level: NodeLevel,
+    /// Set to a non-None value when the formatter is running on a code
+    /// snippet within a docstring. The value should be the quote style of the
+    /// docstring containing the code snippet.
+    ///
+    /// Various parts of the formatter may inspect this state to change how it
+    /// works. For example, multi-line strings will always be written with a
+    /// quote style that is inverted from the one here in order to ensure that
+    /// the formatted Python code will be valid.
+    docstring: Option<QuoteStyle>,
 }
 
 impl<'a> PyFormatContext<'a> {
@@ -20,6 +29,7 @@ impl<'a> PyFormatContext<'a> {
             contents,
             comments,
             node_level: NodeLevel::TopLevel(TopLevelStatementPosition::Other),
+            docstring: None,
         }
     }
 
@@ -43,6 +53,27 @@ impl<'a> PyFormatContext<'a> {
     pub(crate) fn comments(&self) -> &Comments<'a> {
         &self.comments
     }
+
+    /// Returns a non-None value only if the formatter is running on a code
+    /// snippet within a docstring.
+    ///
+    /// The quote style returned corresponds to the quoting used for the
+    /// docstring containing the code snippet currently being formatted.
+    pub(crate) fn docstring(&self) -> Option<QuoteStyle> {
+        self.docstring
+    }
+
+    /// Return a new context suitable for formatting code snippets within a
+    /// docstring.
+    ///
+    /// The quote style given should correspond to the style of quoting used
+    /// for the docstring containing the code snippets.
+    pub(crate) fn in_docstring(self, style: QuoteStyle) -> PyFormatContext<'a> {
+        PyFormatContext {
+            docstring: Some(style),
+            ..self
+        }
+    }
 }
 
 impl FormatContext for PyFormatContext<'_> {