Use custom config for completions plugin

[berberman]

• Remove haskell.completionSnippetsOn
• Use Properties (Regularize custom config of plugins #1576) to define the custom config, including switches of whether to enable snippets and auto extending import lists.

Clients now shoud provide JSON value like:

"ghcide-completions":{
      "globalOn":true,
      "config":{
         "autoExtendOn":true,
         "snippetsOn":true
      },
      "completionOn":true
}

in the section haskell.plugin to control completions plugin.

[michaelpj]

Do we want to call it "ghcide-completions"? ghcide is more-or-less not user-facing any more, perhaps we should try and avoid mentioning the name in user-visible config? Could we just call it "completions"?

[berberman]

The name "ghcide-completions" comes from its descriptor:

haskell-language-server/ghcide/src/Development/IDE/Plugin/HLS/GhcIde.hs

Lines 22 to 29 in b9c6e6c

	descriptors :: [PluginDescriptor IdeState]
	descriptors =
	[ descriptor "ghcide-hover-and-symbols",
	CodeAction.descriptor "ghcide-code-actions",
	Completions.descriptor "ghcide-completions",
	TypeLenses.descriptor "ghcide-type-lenses",
	Notifications.descriptor "ghcide-core"
	]

In my opnion we'd better not introduce new names for the key, but use descriptors to keep things simple. Besides, I don't think it would be great to call the key "completions", because there might be other plugins are providing completion functionality as well.

[jneira]

lgtm

[pepeiborra]

One option is to merge all the "ghcide-" descriptors into a single one and call it simply "HLS" or "haskell"

[berberman]

Currently CustomConfig blocks defining the merge operation of descriptors, which means we need define the custom config for type lenses plugin and completions plugin in one Properties if we merge ghcide descriptors together. This would bring unnecessary couplings, imho.