feat(lsp): improve registry completion suggestions

[kitsonk]

Resolves #10051

Still need to do all the tests, but it works. If folks want to take if for a road test, you can enable a prototype registry server by setting the following in your editor configuration:

{
  "deno.enable": true,
  "deno.suggest.imports.hosts": {
    "https://regv2.deno.dev": true
  }
}

The server "proxies" the information from deno.land/x/. Things registry servers can now implement with v2 are:

• Ability to return partial result sets. This is done in a backwards compatible fashion, where a server can either return a string[] or an JSON object like:

  interface VariableItemsList {
    items: string[];
    isIncomplete?: boolean;
    preselect?: string;
  }

  Where if isIncomplete is true, it indicates that not all the results have been returned and subsequent queries to the "url" field can be sent by the client to narrow down the result set.

• Ability to preselect particular selections, by setting the "preselect" value in the variable list response. This allow a registry, for example, to tell the client to choose mod.ts as the default selection when completing an import (or whatever the server determines is the appropriate preselection.

• Ability to provide additional documentation for a suggestion/completion item by providing a "documentation" endpoint as part of the deno-import-intellisense.json. When the client displays a particular item, it will request the server to resolve the completion item, and if the "documentation" endpoint is set, a request will be made to that completion endpoint. The endpoint should response with a documentation object:

   interface Documentation {
    kind: "markdown" | "plaintext";
    value: string;
  }

Technically, all these changes are non-breaking with v1, and the language server will simply adjust based on the runtime behaviour of the registry. The only real change is that v2 will cause the language server to validate the deno-import-intellisense.json in against the v2 specification.

[bartlomieju]

LGTM, I don't have any comments to the code, but I will let Luca or David approve.

[dsherret]

LGTM