[vscode-graphql-syntax] Add support for GraphQL embedded in Hack files

Summary:
This adds support for GraphQL strings within Hack files, bootstrapped from the
existing PHP version. I followed the steps outlined in the README.

Test Plan:
- `yarn test -u` from package root
- `yarn vsce:package` and install resulting VSIX
- Load `test.hack` in VS Code
- Observe nice syntax highlighting for GraphQL

Author: dunnbobcat

.changeset/twelve-rings-sin.md

@@ -0,0 +1,5 @@
+---
+'vscode-graphql-syntax': minor
+---
+
+Added support for GraphQL embedded in Hack files