Add `get.currentStage()`, `get.currentRound()` and `helpers.isRoundCompleted()`

[Drarig29]

This PR aims to help users handle the following scenarios:

• Get all the matches in the current round:

  const tournamentId = 3;
  const currentStage = await manager.get.currentStage(tournamentId);
  const currentRound = await manager.get.currentRound(currentStage.id);
  // or
  const stageId = 5;
  const currentRound = await manager.get.currentRound(stageId);
  
  const currentRoundMatches = await manager.storage.select('match', { round_id: currentRound.id });

• Know if a round is finished or not:

  const { helpers } = require('brackets-manager')
  const roundMatches = await manager.storage.select('match', { round_id: 3 });
  const isFinished = helpers.isRoundCompleted(roundMatches);