bug#55115: 28.1; Add way to customize Flymake mode line text

[Allen Li]

Is it acceptable to add a way to customize the Flymake mode line text?
Mode line space is valuable and I think many users would want to shorten
"Flymake".

Currently it is generated by

flymake-mode-line-format
-> flymake-mode-line-title
-> flymake--mode-line-title

which hardcodes "Flymake".  I would suggest adding an intermediary
variable/user option that could be easily changed.

In GNU Emacs 28.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.33, cairo 
version 1.17.6)
 of 2022-04-20 built on frederik
Windowing system distributor 'The X.Org Foundation', version 11.0.12101003
System Description: Arch Linux