[vscode-graphql-syntax] Inconsistend highlighting of `extend` keyword

### Current Behavior

In the following examples, the `extend` keyword is highlighted differently:



### Expected Behavior

I would expect the `extend` keyword to be highlighted the same in both cases.

_Looking at [the grammars](https://github.com/graphql/graphiql/blob/main/packages/vscode-graphql-syntax/grammars/graphql.json) I get the feeling the `extend` keyword is not supported at all. In this case I'd expect the `extend` keyword to be supported by syntax highlighting._