Add syntax error when conversion flag does not immediately follow exclamation mark

[dylwil3]

Closes #18671

Note that while this has, I believe, always been invalid syntax, it was reported as a different syntax error until Python 3.12:

Python 3.11:

>>> x = 1
>>> f"{x! s}"
  File "<stdin>", line 1
    f"{x! s}"
             ^
SyntaxError: f-string: invalid conversion character: expected 's', 'r', or 'a'

Python 3.12:

>>> x = 1
>>> f"{x! s}"
  File "<stdin>", line 1
    f"{x! s}"
        ^^^
SyntaxError: f-string: conversion type must come right after the exclamanation mark

[github-actions]

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[MichaReiser]

Hehe, the comment is sort of less precise than the variant name.