style: unify code style

[wenfangdu]

Description of Problem

Currently, many code samples aren't following the same code style which can lead to confusion, e.g. (excerpted from here)

<template>
  <div>
    <p>{{ text }}</p>
    <button v-on:click="$emit('accepted')">OK</button>
  </div>
</template>
<script>
👇 indent
  export default {
    props: ['text'],
    emits: ['accepted']
  }
</script>

<template>
  <button v-on:click="$emit('click', $event)">OK</button>
</template>
<script>
👇 no indent
export default {
  emits: [] // without declared event
}
</script>

Proposed Solution

1. Prettier codebase.
2. Add pre-commit hook for prettier.

Additional Information

This PR has no other changes except the above actions.

[skirtles-code]

Thanks for the PR.

There are problems with the formatting changes in some of the files. In some cases the formatting mattered and information has been lost or corrupted as a result of the changes. I'm not going to attempt to list them all here as reviewing over 100 files isn't really practical.

Instead I'm going to create some smaller PRs that make some of these changes by hand. Once those are merged we can try running Prettier again to see what's left and then make some decisions accordingly.