add Support for `gql` documents in Svelte SFC

> https://github.com/apollographql/apollo-tooling/issues/1084
> **Intended outcome:**
>
> Autocompletion and information on hover in document preceded with a `gql` tag in `.vue` files.
>
> **Actual outcome:**
>
> No autocomplete suggestion nor tooltips.
>
> **How to reproduce the issue:**
>
> Create a `.vue` file (like `MyComp.vue`) in the project, then write
>
> ```
>
> import gql from 'graphql-tag'
>
> const QUERY = gql`
>
> `
>
> ```

Actually, the same would be true for svelte, which would be genuinely nice to have.

Start by reproducing the example in a .vue file, then investigate how gql-tagged documents are currently recognized for editor autocomplete and hover information. Extend the relevant support to Svelte SFCs; done means both autocomplete suggestions and tooltips work for gql-tagged documents in Svelte files.

Written by the indexing model from the issue text.