Add haskell.plugin.fourmolu.config.path option

[georgefst]

Companion to haskell/haskell-language-server#3860. Shouldn't be merged until that is.

The added lines come from manually inspecting the output of cabal run exe:haskell-language-server vscode-extension-schema. Do we have a better automatic process for this (I'm sure it'd be fairly easy to script with jq, perhaps after an initial commit re-ordering things)?

[fendor]

Do we have a better automatic process for this

We don't, unfortunately :(
The output of HLS should be sorted already, so I just copy and paste it in.

What makes this task especially tedious, you need to be careful to run vscode-extension-schema with a HLS binary that supports all plugins, otherwise some entries will be missing, since vscode-extension-schema depends on the installed plugins.
We had an idea to decouple the plugin from its configuration to fix this, but no one got around to it, yet.