Add support for multi-character operator tokens to SimpleTokenizer (#6563)

## Summary

Allows for proper lexing of tokens like `->`.

The main challenge is to ensure that our forward and backwards
representations are the same for cases like `===`. Specifically, we want
that to lex as `==` followed by `=` regardless of whether it's a
forwards or backwards lex. To do so, we identify the range of the
sequential characters (the full span of `===`), lex it forwards, then
return the last token.

## Test Plan

`cargo test`

crates/ruff_python_trivia/src/cursor.rs

@@ -30,6 +30,14 @@ impl<'a> Cursor<'a> {
         self.chars.clone().next().unwrap_or(EOF_CHAR)
     }
 
+    /// Peeks the second character from the input stream without consuming it.
+    /// Returns [`EOF_CHAR`] if the position is past the end of the file.
+    pub fn second(&self) -> char {
+        let mut chars = self.chars.clone();
+        chars.next();
+        chars.next().unwrap_or(EOF_CHAR)
+    }
+
     /// Peeks the next character from the input stream without consuming it.
     /// Returns [`EOF_CHAR`] if the file is at the end of the file.
     pub fn last(&self) -> char {