Update `PLE2510`, `PLE2512-2515` to check in f-strings

`PLE2510`, `PLE2512`, `PLE2513`, `PLE2514`, and `PLE2515` checks for invalid string characters.

The rule needs to be updated to check within the f-string as well. Here, we only need to check within the `FStringMiddle` token which contains the non-expression part of a f-string.