support await keyword

I propose that we support async/await semantics in brighterscript.

example:

      private async function login()
        m.account = await m.getAccount(m.nameInput.text, m.passwordInput.text
        if m.account
          m.username.label.text = m.account.name
        else
          m.username.label.text = "ERROR"
        end if
     end function

    private async function getAccount(name as string, password as string)
      token = await m.getToken(name, password)
      user = await m.getUserAccount(token)
      return user <> invalid ? user.account : invalid
    end function

    private async function getToken(name as string, password as string)
      'returns a promise, as per roku-promise (i.e a creates a task with an out field)
      'e.g.
      task = createObject("roSGNode", "UserTask")
      task.command = "getToken"
      task.name = name
      task.password = password
      task.control = "RUN"
      return task
    end function

    private async function getUserAccount(token as string)
      'returns a promise, as per roku-promise (i.e a task with an out field)
      task = createObject("roSGNOde", "UserTask")
      task.command = "getAccount"
      task.token = token
      task.control = "RUN"
      return task
    end function

The issue names no files, tests, or entry points. Start by clarifying the intended async/await semantics and how the proposed task-based promises should behave; done would require an agreed design, implementation, and coverage for the shown examples.