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_ast/src/lib.rs

@@ -21,6 +21,7 @@ pub mod relocate;
 pub mod statement_visitor;
 pub mod stmt_if;
 pub mod str;
+pub mod str_prefix;
 pub mod traversal;
 pub mod types;
 pub mod visitor;