gh-91520: Rewrite imghdr inlining for clarity and completeness (#91521)

* Rewrite imghdr inlining for clarity and completeness

* Move MIMEImage class back closer to the top of the file since it's the
  important thing.
* Use a decorate to mark a given rule function and simplify the rule function
  names for clarity.
* Copy over all the imghdr test data files into the email package's test data
  directory.  This way when imghdr is actually removed, it won't affect the
  MIMEImage guessing tests.
* Rewrite and extend the MIMEImage tests to test for all supported
  auto-detected MIME image subtypes.
* Remove the now redundant PyBanner048.gif data file.

* See https://github.com/python/cpython/pull/91461#discussion_r850313336

Co-authored-by: Oleg Iarygin <dralife@yandex.ru>

Co-authored-by: Oleg Iarygin <dralife@yandex.ru>