Add eat_char2 for the lexer (#6968)

## Summary

This PR adds a new helper method on the `Cursor` called `eat_char2`
which is similar to `eat_char` but accepts 2 characters instead of 1. It'll
`bump` the cursor twice if both characters are found on lookahead.

## Test Plan

`cargo test`

crates/ruff_python_parser/src/lexer/cursor.rs

@@ -85,6 +85,17 @@ impl<'a> Cursor<'a> {
         }
     }
 
+    pub(super) fn eat_char2(&mut self, c1: char, c2: char) -> bool {
+        let mut chars = self.chars.clone();
+        if chars.next() == Some(c1) && chars.next() == Some(c2) {
+            self.bump();
+            self.bump();
+            true
+        } else {
+            false
+        }
+    }
+
     pub(super) fn eat_if<F>(&mut self, mut predicate: F) -> Option<char>
     where
         F: FnMut(char) -> bool,