W605 checks for invalid escape sequences in a string.

For f-strings, we'll be looking for the same in a FStringMiddle token which is the non-expression part of the f-string. The rule extracts the body part which excludes the quotes and also skips the validation if it's a raw string:

The FStringMiddle token contains both the above information for free. We just need to check if the token is a FStringMiddle or a String.