Rule proposal: v-for-delimiter-style

[FloEdelmann]

Please describe what the rule should do:
You can use both v-for="x in xs" and v-for="x of xs". There should be a rule to enforce one of the styles (probably configurable).

What category should the rule belong to?

• Enforces code style
• Warns about a potential error
• Suggests an alternate way of doing something
• Other (please specify:)

Provide 2-3 code examples that this rule should warn about:

<!-- "vue/v-for-delimiter-style": ["warn", "in"] -->
<div v-for="x in xs" /><!-- GOOD -->
<div v-for="x of xs" /><!-- BAD -->

<!-- "vue/v-for-delimiter-style": ["warn", "of"] -->
<div v-for="x of xs" /><!-- GOOD -->
<div v-for="x in xs" /><!-- BAD -->

Additional context
See https://vuejs.org/v2/guide/list.html#Mapping-an-Array-to-Elements-with-v-for:

You can also use of as the delimiter instead of in, so that it is closer to JavaScript’s syntax for iterators.

[ota-meshi]

Thank you for your proposal!

Sounds good to me.

However, looking at the rules of eslint, it seems that the meaning is a little different from the rules that have consistent as a prefix.
I think names like vue/v-for-delimiter or vue/v-for-delimiter-style are good. What do you think?

[FloEdelmann]

Indeed. I'd go with vue/v-for-delimiter-style then. I've updated the issue title and description.

[ota-meshi]

If the PR to the parser is unacceptable, you can get it from the token. (it's difficult to read...)

const tokenStore =
    context.parserServices.getTemplateBodyTokenStore &&
    context.parserServices.getTemplateBodyTokenStore()

const token = /** @type {Token} */ (tokenStore.getTokenAfter(
  node.left.length ? node.left[node.left.length-1] : tokenStore.getFirstToken(node),
  token => token.type !== 'Punctuator' || token.value !== ')'
))

token.value // "in" or "of"

The following sources are helpful.

eslint-plugin-vue/lib/utils/indent-common.js

Line 1218 in a2614ef

const inToken = /** @type {Token} */ (tokenStore.getTokenAfter(