PrefixAll: add service prefix

[brawaru]

For PrefixAl' core module, add service prefix which can only be used by the bot owner.

Service prefix must be configurable, but may have a default value of…

// ❄ Snowball Bot

function getPrefix() {
	const tag = $botConfig.name
		.match(/[a-z_\-0-9 ]+/i)[0]
			.trim()
			.replace(/ /g, "_")
			.toLowerCase();

	return `${tag}:`;
}

// snowball_bot:

This spaghetti above returns good complex prefix which with big change will not be used by any other bot.

Examples of commands with this prefix:

• snowball_bot:change_avatar
• snowball_bot:change_name
• snowball_bot:ping

Good, isn't it?