Support for GraphQL template highlighting

[tgriesser]

Hello! Just getting started with Vue and ran into an issue getting GraphQL syntax picked up within the <script> blocks using Volar. The same feature works with Vetur, so if you had any ideas on the best way to add support this I could work on a PR.

Here's a potentially relevant issue in Vetur: vuejs/vetur#701 (comment) and here's where the graphql-vscode plugin defines support for vue.

Volar	Vetur

Thanks!

[djmtype]

Please also support graphql highlighting within Gridsome's <page-query> and <static-query> too.