Lint GraphQL queries with `eslint-plugin-graphql`

GraphQL queries written on the client-side are currently not validated in our build process. This can lead to bugs e.g.:

• querying for something that doesn't exist
• duplicate query definitions

We should be able to leverage eslint-plugin-graphql to lint our .graphql files against our schema.

Why?

The addition of such a set of tooling would:

• boost productivity of anyone writing GraphQL queries in gitlab-org/gitlab
• reduce code review duration when .graphql files are involved
• reduce occurrence of bugs

How?

@tomquirk briefly explored this in !33242 (closed), however came across some issues listed in the section below

Issues that need to be resolved

• Some of our queries contain a @client directive (e.g. https://gitlab.com/gitlab-org/gitlab/blob/c97c456dfa878acdc755f97326d40f77d5f4bee4/app/assets/javascripts/design_management/graphql/queries/active_discussion.query.graphql#L4), and eslint-plugin-graphql can't seem to handle this case out of the box.
• eslint-plugin-graphql doesn't seem to catch duplicate query definitions, whereas the Apollo extension for VS Code does