Cannot add meta types or fields to schema

**Describe the bug**
As described in https://github.com/dotansimha/graphql-code-generator/issues/2911#issuecomment-561019738 and https://github.com/dotansimha/graphql-code-generator/issues/3459#issuecomment-583487477, we're supposed to add meta types and fields to the schema ourselves for use with codegen. However that doesn't work as the schema fails validation:

> Error: Name "\_\_schema" must not begin with "\_\_", which is reserved by GraphQL introspection.

**To Reproduce**
Steps to reproduce the behavior:

1. My GraphQL schema:

```graphql
type Query {
__schema: __Schema!
}
```

2. My GraphQL operations:

```graphql
query Query {
__schema {
__typename
}
}
```

3. My `codegen.yml` config file:

```yml
schema:
- src/schema.graphql

documents:
- src/document.graphql

generates:
src/types.ts:
plugins:
- typescript
```

**Expected behavior**

Types successfully generated for the meta query.

**Environment:**

- OS: macOS Catalina 10.15.1
- ```json
"@graphql-codegen/cli": "^1.12.2",
"@graphql-codegen/typescript": "^1.12.2",
```
- NodeJS: 12.14.1