Move prettier to be a peer dependency

[BPScott]

Helping along with https://github.com/Shopify/sewing-kit/issues/737. We should strive to have a single prettier install underneath sewing-kit, rather than multiple dependencies potentially installing multiple prettier versions which leads to confusion around different formatting tools producing different results.

eslint-plugin-prettier expects prettier to be a peer dependency, so do the same here.

[BPScott]

Currently running yarn why prettier in web returns:

info Reasons this module exists
   - "@shopify#sewing-kit" depends on it
   - Hoisted from "@shopify#sewing-kit#prettier"
   - Hoisted from "@shopify#sewing-kit#eslint-plugin-shopify#prettier"
   - Hoisted from "@shopify#sewing-kit#prettier-stylelint-formatter#prettier"
   - Hoisted from "@shopify#sewing-kit#stylelint-config-shopify#prettier-stylelint-formatter#prettier"

This should remove the Hoisted from "@shopify#sewing-kit#eslint-plugin-shopify#prettier" line. See the linked issue for PRs relating to removing the others

[ismail-syed]

I believe this is the right call here.

We had discussion about this in the past. We had prettier set as an optional dependency because consumers may use eslint-plugin-shopify without the use of prettier.

However, docs on [optionalDependencies](https://docs.npmjs.com/files/package.json#optionaldependencies) seem to imply different behaviour.

The difference is that build failures do not cause installation to fail.

It doesn't seem to have much impact on how dependency resolution works with regards to devDependencies vs optionalDependencies. IIRC, when I first PR'd this, I had it set as a peerDep, then changed it to an optionalDep after PR feedback.

[ismail-syed]

We should update the readme to let consumers know that prettier should be installed separately as a peerDep.

[GoodForOneFare]

👍 Needs a CHANGELOG entry, ideally highlighting that this is a breaking change.

[ismail-syed]

Can we also add a note here for consumers to install prettier themselves. Frequent visitors are more likely to read the changelog over readme history.