Refactor slack config

[Yukaii]

zuzugo/lib/config.ts

Lines 40 to 49 in a2222fa

	slackWebhook,
	slackAppToken: process.env.SLACK_APP_TOKEN,
	slackBotToken: process.env.SLACK_BOT_TOKEN,
	slackSigningSecret: process.env.SLACK_SIGNING_SECRET,
	slackDevMode: process.env.SLACK_DEV_MODE === "true",
	slackClientId: process.env.SLACK_CLIENT_ID,
	slackClientSecret: process.env.SLACK_CLIENT_SECRET,
	slackStateSecret: process.env.SLACK_STATE_SECRET,
	slackRedirectUri: process.env.SLACK_REDIRECT_URI,
	slackSlashCommand: process.env.SLACK_SLASH_COMMAND,

Refactor to scoped config like:

slack: {
  incomingWebhook: process.env.INCOMING_WEBHOOK,
  ...
}