Move token and error structs into related modules (#11957)

## Summary

This PR does some housekeeping into moving certain structs into related
modules. Specifically,
1. Move `LexicalError` from `lexer.rs` to `error.rs` which also contains
the `ParseError`
2. Move `Token`, `TokenFlags` and `TokenValue` from `lexer.rs` to
`token.rs`

crates/ruff_python_parser/src/parser/statement.rs

@@ -8,14 +8,14 @@ use ruff_python_ast::{
 };
 use ruff_text_size::{Ranged, TextSize};
 
-use crate::lexer::TokenValue;
 use crate::parser::expression::{ParsedExpr, EXPR_SET};
 use crate::parser::progress::ParserProgress;
 use crate::parser::{
     helpers, FunctionKind, Parser, RecoveryContext, RecoveryContextKind, WithItemKind,
 };
+use crate::token::{TokenKind, TokenValue};
 use crate::token_set::TokenSet;
-use crate::{Mode, ParseErrorType, TokenKind};
+use crate::{Mode, ParseErrorType};
 
 use super::expression::ExpressionContext;
 use super::Parenthesized;