Add syntax error when conversion flag does not immediately follow exclamation mark (#18706)

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:

```pycon
>>> 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:

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

crates/ruff_python_parser/resources/inline/err/f_string_conversion_follows_exclamation.py

@@ -0,0 +1,3 @@
+f"{x! s}"
+t"{x! s}"
+f"{x! z}"