[Fixes #294] Fix no-unused-vars rule

[michalsnik]

This PR fixes no-unused-vars rule, so it doesn't throw a warning, when some of the properties are not used even when the last property was used.

As pointed in #294 currently code like this throws warnings:

<template>
  <!-- 'v'  is defined but never used, 'i' ..., 'c' ... -->
  <div v-for="(v, i, c) in foo">{{c}}</div>
</template>

In this PR I'm checking if the last property was referenced anywhere. If yes then we simply ignore other properties, if not -> we check references of all properties as before.

[mysticatea]

As it happens, there is a discussion that core no-unused-vars rule should report all unused variables which are after the last used variable: eslint/eslint#9750

I like that approach.
Would you change this PR by using a for loop which goes from last to first?

for (let i = node.variables.length - 1; i >= 0 && node.varibles[i].references.length === 0; --i) {
  // report it.
}

[michalsnik]

I think this PR introduces exactly that @mysticatea, it works as follows:

<template>
  <div v-for="(v, i, c) in foo">{{v}}</div>
</template>

☝️ reports: i and c

<template>
  <div v-for="(v, i, c) in foo">{{i}}</div>
</template>

☝️ reports: c

<template>
  <div v-for="(v, i, c) in foo">{{c}}</div>
</template>

☝️ reports nothing

[mysticatea]

You are wrong in the 2nd case.

[michalsnik]

Holly molly! You're absolutely right. Thank you for being so vigilant! I'll update it tomorrow :)

[michalsnik]

Should be good now @mysticatea

[mysticatea]

LGTM, thank you!