Disable reloading when changing language servers

[kimadeline]

For #18509

Behaviour before this PR:

• Language server-specific code registered on extension activation
• Language server check (if default value, if Python 2.7) done on language server start during activation
• Changing the value of the python.languageServer setting would trigger a prompt asking to reload

Behaviour in this PR:

• The only LS-related class registered on activation is LanguageServerWatcher
• The LS watcher replaces the LanguageServerExtensionActivationService
• The watcher listens to settings changes and interpreter changes, will start the relevant language server and dispose of the existing language server (if any)
• In order to spin up the language server, the watcher will instantiate an object implementing the ILanguageServerExtensionManager interface for that specific language server, and then call ILanguageServerExtensionManager.startLanguageServer
• The ILanguageServerExtensionManager classes replace the Jedi and Pylance activator classes, with some added functionality to start up and dispose of the language server classes
• LS-specific classes are not injectable anymore, and all their dependencies are explicitly listed in their constructor, which means that they have gotten pretty big 😔
• This PR also removes the NodeLanguageServerFolderService class, which was a remnant from the old MPLS days. It had a few "convenience" functions that were doing way too much work to infer the LS version, when we can pick it up using the VS Code API.

General architecture

classDiagram
      class ILanguageServerCapabilities {
          ILanguageServerProxy serverProxy
          Promise<ILanguageServer> get()
      }
      class ILanguageServerExtensionManager {
          Promise<void> startLanguageServer()
          void stopLanguageServer()
          boolean canStartLanguageServer()
          void languageServerNotAvailable()
          void dispose()
      }         
      ILanguageServerCapabilities <|-- LanguageServerCapabilities
      ILanguageServerCapabilities <|-- ILanguageServerExtensionManager
      LanguageServerCapabilities <|-- JediLSExtensionManager
      LanguageServerCapabilities <|-- PylanceLSExtensionManager
      ILanguageServerExtensionManager <|-- NoneLSExtensionManager
      ILanguageServerExtensionManager <|-- JediLSExtensionManager
      ILanguageServerExtensionManager <|-- PylanceLSExtensionManager
      class JediLSExtensionManager{
      }
      class PylanceLSExtensionManager {
      }
      class NoneLSExtensionManager {
      }

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Activation flow

flowchart TD
    id1["Extension load"]--->|Register watcher singleton|id2["Activate services"]--->|"watcher.startLanguageServer()"|id3["watcher.createLanguageServer(lsType)"]--->|"LS extension manager created"|id4["lsExtensionManager.startLanguageServer()"]

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[karrtikr]

Code appears way simpler now 🥳 great work. I have some concerns around the support for multiroot scenario, especially for Jedi.

[karrtikr]

LGTM

[karrtikr]

Suggested change

	): Promise<ILanguageServerExtensionManager>;
	): Promise<void>;

Given it's not used anywhere outside the class, do we want to expose methods of ILanguageServerExtensionManager outside? Something to think about.

[kimadeline]

A subset of ILanguageServerExtensionManager methods are exposed on purpose for the classes implementing this interface.

I'll add to the comment above the definition of ILanguageServerExtensionManager that these methods shouldn't be used.

[karrtikr]

classes implementing this interface.

I understand they're exposed for the LanguageServerWatcher which is the class implementing ILanguageServerWatcher, it's fine to expose it for internal use.

But I don't think we need to expose ILanguageServerExtensionManager for external use (code outside language servers), so returning void here should be sufficient.

[kimadeline]

Will not address this for now, captured in #18946.