Add a way (config option) to disable language services

[dhruvmanila]

We should provide a way for users to disable language services like completions, goto definition, etc. until we've those capabilities implemented in the language server.

There's some precedence to this:

• pyright.disableLanguageService

  pyright.disableLanguageServices [boolean]: Disables all language services. This includes hover text, type completion, signature completion, find definition, find references, etc. This option is useful if you want to use pyright only as a type checker but want to run another Python language server for language service features.

• python.pyrefly.disableLanguageService

  If true, pyrefly will only provide typechecking in the IDE. It will not provide other IDE services like completions, hover, definition, etc.

Maybe this would be better than the experimental namespace that exists currently? But this would be similar to --preview flag in that it will include all capabilities compared to the current experimental.completions.enable which allows users to enable / disable specific features.

This issue is mainly to start a discussion and not to implement the mentioned config option.