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/mod.rs

@@ -5,9 +5,9 @@ use bitflags::bitflags;
 use ruff_python_ast::{Mod, ModExpression, ModModule};
 use ruff_text_size::{Ranged, TextRange, TextSize};
 
-use crate::lexer::TokenValue;
 use crate::parser::expression::ExpressionContext;
 use crate::parser::progress::{ParserProgress, TokenId};
+use crate::token::TokenValue;
 use crate::token_set::TokenSet;
 use crate::token_source::{TokenSource, TokenSourceCheckpoint};
 use crate::{Mode, ParseError, ParseErrorType, TokenKind};