Add eat_char2 for the lexer

[dhruvmanila]

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

[dhruvmanila]

Current dependencies on/for this PR:

• main
  • PR Add eat_char2 for the lexer #6968 👈
    • PR Add support for the new f-string tokens per PEP 701 #6659

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PR Check Results

Ecosystem

✅ ecosystem check detected no changes.