Rework Plugin API (descriptions, completions...)

[pomdtr]

Idea: take inspiration from raycast script commands

#!/bin/bash

# Required parameters:
# @raycast.schemaVersion 1
# @raycast.title Open Pull Requests page
# @raycast.mode silent
#
# Optional parameters:
# @raycast.packageName GitHub
# @raycast.icon images/github-logo.png
# @raycast.iconDark images/github-logo-iconDark.png

open https://github.com/pulls/