Add autoload cookie for prettier-js

[dakra]

I never use prettier-mode but always manually call prettier-js.

This adds an autoload cookie for this command so it's available when the library is not loaded yet.

[jrgtt]

Came here to add this, glad someone already thought about it.

[leodag]

Also came here for this - would love for it to be merged.

[dakra]

Since there is not much happening in this repo I would suggest https://github.com/raxod502/apheleia which is what I use currently and it's pretty nice.