feat: upgrade prettier

[FauxFaux]

Literally just bump prettier.

Unfortunately, this brings with it some new defaults, and a load of irrelevant changes. If you want, you can disable trailingComma in .prettierrc, instead?

This is useful to me because it reduces the number of copies of prettier I have to install, as optional in npm doesn't actually mean optional.

[haoqunjiang]

See #84 (comment)

Bumping prettier version would mean breaking changes for a whole lot of Vue core packages, including vue-loader and @vue/cli. And as you can see, vue-loader v16 is aimed for Vue 3 support, so we are not going to ship yet another major version for such niche use cases.

In the foreseeable future, I think the only viable solution is to make prettier an optional peer dependency. It would still mean breaking change in this package, though. So might better ship it along with some other planned breaking changes so as not to waste a major version number.