[language-service] parsing error for string template variables in a "default argument value" position

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

I'd love to be able to extract default argument values into constants, so I can:

* avoid "magic values" in the schema
* use the constant default values inside other functions

This is the pattern I'd like to implement:

```ts
const schema = gql`
type ImageAsset {
src(
height: Int
width: Int = ${DEFAULT_WIDTH}
): ImageSource!
}
`;
```

... which unfortunately leads to a parsing error: `Syntax Error: Unexpected ")".`



### Expected Behavior

`vscode-graphql` should understand that this is, at the very least, "some default value" (understandable that it probably won't be able to grab the exact value — that would be pretty awesome, though!)

### Steps To Reproduce

```ts
const schema = gql`
type ImageAsset {
src(
height: Int
width: Int = ${DEFAULT_WIDTH}
): ImageSource!
}
`;
```

### Environment

```Markdown
- VSCode extension `graphql.vscode-graphql` Version: 0.7.5
- OS: MacOS Ventura (13.0 Beta, build 22A5352e)
```

### Anything else?

_No response_