Use typescript info for interpolation completion

[ktsn]

fix #1129

To use ts.LanguageService#getCompletionsAtPosition to get completion list for interpolation expression. It also combines previous completion info so that the behavior should be backward compatible.

There is a known issue when an interpolation expression is invalid. Our source map cannot handle expression position, then provides wrong completion in that case. We need to think how we handle invalid expression in the future.
-> Fixed via #1448

<template>
  <div>{{ obj. }}</div> <!-- Cannot provide `obj.value` completion -->
</template>

<script lang="ts">
import Vue from 'vue'

export default Vue.extend({
  data() {
    return {
      obj: { value: '' }
    }
  }
})
</script>

[sabifa]

I would love to see that feature! Could we get a small update about it, please? 😄

[octref]

I got completion resolve as well:

[octref]

Still need to fix the empty completion case {{ | }} but this is much better now.

[octref]

Those are the only things I'm not haapy about. Other than that this seems good so I'm merging.

[octref]

I had to change this because otherwise the typing would be wrong and cause the whole this to have any type...

[ktsn]

It's probably because of the return type annotation problem... 😢
https://vuejs.org/v2/guide/typescript.html#Annotating-Return-Types

[octref]

JSDocs return type doesn't work either. Sad..

[octref]

I had to change this because otherwise the typing would be wrong and cause the whole this to have any type...