feat(task): add ability to specify task description

[bartlomieju]

Closes #14949

Allows to specify tasks using object notation and include a
description for a task:

{
  "tasks": {
    "echo": "echo 1",
    "eval": {
       "command": "deno eval 'console.log(3)'",
       "description": "Print value 3"
    }
}

$ deno task
Available tasks:
- echo
  echo 1
- eval Print value 3
  deno eval 'console.log(3)'

[bartlomieju]

Do not merge need to update schema file still