Automatically exclude `node_modules/` from `documents`

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

If you install `graphql-codegen` at the root of a monorepo, and then specify `documents: ["**/*.graphql"]`, you will accidentally process every document twice - once locally, once again as a child of `node_modules/`.

**Describe the solution you'd like**

`tsc` automatically excludes `node_modules/`. So does `eslint`. I think `graphql-codegen` should do the same. I can't imagine any valid reason to include it by default.

**Describe alternatives you've considered**

For now, I've added `"!node_modules/"` to my `documents`. But I think this should be built-in functionality to match the rest of the ecosystem.

**Additional context**