Allow HLS plugins to declare cli commands

[pepeiborra]

I want to add a command line retrie subcommand provided by hls-retrie-plugin. Generally, this PR enables any PluginDescriptor to declare new cli commands.

For consistency, I moved --vscode-extension-schema and --generate-default-config to be subcommands as well. Let me know if anything else needs changing; where are these commands called from?

How it looks like:

Available commands:
  typecheck                Used as a test bed to check your IDE will work
  hiedb                    Query .hie files
  lsp                      Start talking to an LSP client
  vscode-extension-schema  Print generic config schema for plugins (used in the
                           package.json of haskell vscode extension)
  generate-default-config  Print config supported by the server with default
                           values
  retrie                   Retrie commands

[pepeiborra]

/cc @xich

[berberman]

LGTM. As far as I know we use --vscode-extension-schema and --generate-default-config manually to update the package.json of our vscode extension when needed. I think we could create a command config and let these two commands be subcommands, grouping them together.