chore: enforce styling across the repo

[xfq]

Description of Problem

There are some different auto-formatting styles in some PRs.

Proposed Solution

Add a pre-commit hook.

Additional Information

See https://github.com/vuejs/docs/issues/233

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[skirtles-code]

We've experimented with using automatic formatting before, but it's always proven to be problematic. The examples in the docs sometimes need the flexibility to deviate from strict formatting rules to allow for more effective communication.

I just tried running Prettier over the existing files and it does find some inconsistencies that need to be fixed. But it also changes things that really shouldn't be changed. While there are some weaker styling rules that could theoretically be applied, I don't think we'd be able to achieve that using Prettier.

I'm going to leave this open for now, as we need to fix some of the actual mistakes that Prettier finds.

[yyx990803]

For the reason explained - some examples are easier to understand when hand-formatted.