Move string-prefix enumerations to a separate submodule (#11425)

## Summary

This moves the string-prefix enumerations in `ruff_python_ast` to a
separate submodule. I think this helps clarify that these prefixes are
purely abstract: they only depend on each other, and do not depend on
any of the other code in `nodes.rs` in any way. Moreover, while various
AST nodes _use_ them, they're not really nodes themselves, so they feel
slightly out of place in `nodes.rs`.

I considered moving all of them to `str.rs`, but it felt like enough
code that it could be a separate submodule.

## Test Plan

`cargo test`

crates/ruff_python_parser/src/lexer.rs

@@ -35,7 +35,9 @@ use unicode_ident::{is_xid_continue, is_xid_start};
 use unicode_normalization::UnicodeNormalization;
 
 use ruff_python_ast::{
-    str::Quote, AnyStringFlags, AnyStringPrefix, FStringPrefix, Int, IpyEscapeKind,
+    str::Quote,
+    str_prefix::{AnyStringPrefix, FStringPrefix},
+    AnyStringFlags, Int, IpyEscapeKind,
 };
 use ruff_text_size::{TextLen, TextRange, TextSize};