refactor: Make Parser methods immutable using interior mutability

Changed all parsing methods to take '&self' instead of '\&mut self'.
Mutable parser state (token index and parser state) now uses
for interior mutability.

This refactoring is preparation for the borrowed tokenizer work. When
holding borrowed tokens from the parser (with lifetime tied to '\&self'),
we cannot call methods requiring '\&mut self' due to Rust's borrowing
rules. Using interior mutability resolves this conflict by allowing
state mutations through shared references.

tests/sqlparser_custom_dialect.rs

@@ -39,7 +39,7 @@ fn custom_prefix_parser() -> Result<(), ParserError> {
             is_identifier_part(ch)
         }
 
-        fn parse_prefix(&self, parser: &mut Parser) -> Option<Result<Expr, ParserError>> {
+        fn parse_prefix(&self, parser: &Parser) -> Option<Result<Expr, ParserError>> {
             if parser.consume_token(&Token::Number("1".to_string(), false)) {
                 Some(Ok(Expr::Value(Value::Null.with_empty_span())))
             } else {
@@ -72,7 +72,7 @@ fn custom_infix_parser() -> Result<(), ParserError> {
 
         fn parse_infix(
             &self,
-            parser: &mut Parser,
+            parser: &Parser,
             expr: &Expr,
             _precedence: u8,
         ) -> Option<Result<Expr, ParserError>> {
@@ -110,7 +110,7 @@ fn custom_statement_parser() -> Result<(), ParserError> {
             is_identifier_part(ch)
         }
 
-        fn parse_statement(&self, parser: &mut Parser) -> Option<Result<Statement, ParserError>> {
+        fn parse_statement(&self, parser: &Parser) -> Option<Result<Statement, ParserError>> {
             if parser.parse_keyword(Keyword::SELECT) {
                 for _ in 0..3 {
                     let _ = parser.next_token();