Generated `TypedDocumentString` generic params improvement

### Is your feature request related to a problem? Please describe.

Currently, generated code including `TypedDocumentString` contains ugly and seemingly unnecessary type assertions. For example:

```ts
export const ProductsGetListDocument = new TypedDocumentString(`
query ProductsGetList {
products(first: 10) {
id
name
}
}
`) as unknown as TypedDocumentString;
```

### Describe the solution you'd like

The assertion can be omitted and generic parameters could be added directly after `new TypedDocumentString` like this:

```ts
export const ProductsGetListDocument = new TypedDocumentString(`
query ProductsGetList {
products(first: 10) {
id
name
}
}
`);
```

> **Note**
> I have a [very rough version of this implemented on my fork](https://github.com/dotansimha/graphql-code-generator/compare/master...typeofweb:graphql-code-generator:TypedDocumentString?expand=1). Could open a PR.

### Is your feature request related to a problem? Please describe.

It's not related to a problem _per se_ but generated code could be made prettier and more concise.