Reduce memory usage of Docstring struct (#16183)

crates/ruff_python_ast/src/str_prefix.rs

@@ -1,3 +1,5 @@
+use ruff_text_size::TextSize;
+
 use std::fmt;
 
 /// Enumerations of the valid prefixes a string literal can have.
@@ -33,6 +35,13 @@ impl StringLiteralPrefix {
             Self::Raw { uppercase: false } => "r",
         }
     }
+
+    pub const fn text_len(self) -> TextSize {
+        match self {
+            Self::Empty => TextSize::new(0),
+            Self::Unicode | Self::Raw { .. } => TextSize::new(1),
+        }
+    }
 }
 
 impl fmt::Display for StringLiteralPrefix {