feat(completions): Add LSP, WebSocket-based autocompletion logic

[S-furi]

This PR introduces an alternative way to retrieve autocompletions from compiler server through WebSocket (see KTL-2807 and JetBrains/kotlin-compiler-server#878).

Note: This feature will be disabled by default as by now it is just an alternative and experimental.

The completion retrieval implementation is backed by the built-in WebSocket API.

The protcol between the server and the client consists of 3 kind of frames:

• init sent on connection, responses will look like Init { sessionId: string, requestId: string? }
• Completion requests are triggered from the client specifying a requestId (a random UUID) with:

{
  "requestId": "string",
  "completionRequest": {
    "files": [
      {
        "name": "string",
        "text": "string"
      }
    ]
  },
  "line": "int",
  "ch": "int"
}

• For each completion request a client could either get one of the following frames:
  • Error: { requestId: string?, message: string } in case an error occurred (note: requestId might be empty for errors like a malformed request).
  • Discarded: { requestId: string? } sent as a backpressure mechanism: it informs the client that the request with the associated requestId has been dropped in favor of a more recent request from the same client for the same document.
  • A completion result as follows:

{
  "requestId": "string",
  "completions": [
    {
      "text": "string",
      "displayText": "string",
      "tail": "string?",
      "import": "string?",
      "icon": "Icon?",
      "hasOtherImport": "boolean?"
    }
  ]
}