Here's an updated version of my previous patch which avoids the need for
a new variant in the defcustom.  When flymake-indicator-type is set to
fringes and we're on a text frame, we simply fall back to using margins.
That allows it to be set to fringes by default without losing
functionality for text terminals; this is how this should have been
implemented originally.

Can we make progress on at least installing this on master, so we can
test it before backporting it to Emacs 30.2?