The task is to update the lexer to emit the new tokens for PEP 701: FStringStart, FStringMiddle and FStringEnd. Along with these, a new token Exclamation needs to be added for conversion flag (f"{foo!s}") as it's now part of the expression.

Some of the error handling which was previously done in the parser will need to be moved into the lexer.

• UnterminatedString
• UnterminatedTripleQuotedString
• UnclosedLbrace - special case as implemented in the CPython tokenizer
• SingleRbrace