Constructs a CustomExecution task object. The callback will be executed when the task is run, at which point the extension should return the Pseudoterminal it will "run in". The task should wait to do further execution until Pseudoterminal.open is called. Task cancellation should be handled using Pseudoterminal.close. When the task is complete fire Pseudoterminal.onDidClose.
The callback that will be called when the task is started by a user. Any ${} style variables that
were in the task definition will be resolved and passed into the callback as resolvedDefinition.
Class used to execute an extension callback as a task.