Mention Zed is supported in the README

[alt-romes]

Someone has kindly prepared support for Zed in the Zed Haskell Extension.
It'd be good to reference it from the readme (Along with support for VScode, vim, emacs)

[brandonchinn178]

ref:

• Add debug adapter for haskell-debugger zed-extensions/haskell#12
• Add support for haskell-debugger zed-extensions/haskell#33

[alt-romes]

It has been merged upstream to Zed in zed-extensions/haskell#33

Great opportunity to reference it from the readme!