Support scoped slot on template diagnostic

It currently just ignores slot-scope and v-slot. We need to add declared variables by scoped slot in local scope.

<template>
  <MyComp v-slot="{ foo }"> <!-- foo will be in local scope -->
    <p>{{ foo }}</p> <!-- should not provide error here -->
  </MyComp>
</template>

[ktsn]

I already made a PR #1210 which supports both v-slot and slot-scope.

[mrg0lden]

I still get an error when I use slot variables for v-bind:class