gh-93096: Make mimetypes CLI tool public

[arhadthedev]

• Document python -m mimetypes (like https://docs.python.org/3/library/ast.html#command-line-usage and https://docs.python.org/3/library/trace.html#command-line-usage do) since it's a solid crossplatform utility
• replace getopt with argparse for unified help text and error messages
• use sys.exit() for error messages
• make tests to call python -m mimetypes with proper checks of stdout, stderr and return codes

gh-93096

[sobolevn]

@arhadthedev do you have time to finish it any time soon? :)

[maxnikulin]

Notice that IANA currently uses the "media types" term:

[RFC2046] specifies that Media Types (formerly known as MIME types) and Media
Subtypes will be assigned and listed by the IANA.

Just an idea, feel free to ignore it: sometimes the following approach may be used to estimate --help message. Try to read man page generated by help2man. Perhaps epilog may be added to ArgumentParser constructor, however I am unsure concerning recommendations within the Python project.

[hugovk]

@arhadthedev Thank you for your work!

[picnixz]

Not sure if it's my configuration or not, but test_guess_type_conflicting_with_mimetypes fails on my openSUSE 15.5. I think the reason is that my /etc/mime.types contains:

image/x-pict pict pict1 pict2 pic pct 

[hugovk]

Right, from the commits looks like it took a while for @arhadthedev to find a good candidate. Let's update it.

@picnixz Can you find another that's not in your /etc/mime.types?

[picnixz]

I'm no more on my Linux session, but I'll do it tomorrow. Can't we mock the array containing the files where everything is looked up?

[hugovk]

Yes, that sounds better than having to worry what different systems happen to have.