Implement FormatPatternMatchValue (#6799)

## Summary

This is effectively #6608, but with additional tests.

We aren't properly handling parenthesized patterns, but that needs to be
dealt with separately as it's somewhat involved.

Closes #6555

crates/ruff_python_formatter/src/pattern/pattern_match_value.rs

@@ -1,19 +1,14 @@
-use ruff_formatter::{write, Buffer, FormatResult};
 use ruff_python_ast::PatternMatchValue;
 
-use crate::{not_yet_implemented_custom_text, FormatNodeRule, PyFormatter};
+use crate::prelude::*;
 
 #[derive(Default)]
 pub struct FormatPatternMatchValue;
 
 impl FormatNodeRule<PatternMatchValue> for FormatPatternMatchValue {
     fn fmt_fields(&self, item: &PatternMatchValue, f: &mut PyFormatter) -> FormatResult<()> {
-        write!(
-            f,
-            [not_yet_implemented_custom_text(
-                "\"NOT_YET_IMPLEMENTED_PatternMatchValue\"",
-                item
-            )]
-        )
+        // TODO(charlie): Avoid double parentheses for parenthesized top-level `PatternMatchValue`.
+        let PatternMatchValue { value, range: _ } = item;
+        value.format().fmt(f)
     }
 }