make async913 disabled by default

[jakkdl]

oopsies

[Zac-HD]

Oops, should have noticed in review 😅

[jakkdl]

...although looking at it more closely, we already default --disable to ASYNC9, and what disable_codes_by_default achieves is adding the codes to --ignore. So in effect we're doubly disabling the codes and you have to both --select and --enable them to get run? Seems bad, we probably shouldn't bother with calling
...nvm nope! Turns out we change the default to --disable depending on if we're standalone or not.

This is a jolly mess, which maaybe made sense before flake8 started enforcing three-letter codes in --ignore (and presumably --select). We probably should stop calling option_manager.extend_default_ignore(default_disabled_error_codes) and make --disable always have the default of ASYNC9, and lean into that end-users should only use enable/disable, and never try to use select/ignore.

edit: opened #260 for discussion of the issue

[jakkdl]

should've marked it draft while writing up that comment x)