GraphQLError: Syntax Error: Expected "schema", found String "Represents a schema"

**Intended outcome:**

Extension works

**Actual outcome:**

```
GraphQLError: Syntax Error: Expected "schema", found String "Represents a schema"
at syntaxError (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/error/syntaxError.js:15:10)
at Parser.expectKeyword (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/language/parser.js:1435:42)
at Parser.parseSchemaDefinition (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/language/parser.js:738:10)
at Parser.parseTypeSystemDefinition (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/language/parser.js:690:23)
at Parser.parseDefinition (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/language/parser.js:154:19)
at Parser.many (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/language/parser.js:1518:26)
at Parser.parseDocument (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/language/parser.js:111:25)
at Object.parse (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/node_modules/graphql/language/parser.js:36:17)
at FileSchemaProvider.loadFileAndGetDocument (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/lib/language-server/providers/schema/file.js:57:30)
at FileSchemaProvider.resolveSchema (/home/codespace/.vscode-remote/extensions/apollographql.vscode-apollo-1.19.6/lib/language-server/providers/schema/file.js:23:21) {
locations: [ { line: 1, column: 1 } ]
}
```

**How to reproduce the issue:**

1. Use elixir absinthe to generate a schema document
2. Start vscode

**Versions**

Here's the schema:

```
"Represents a schema"
schema {
query: RootQueryType
mutation: RootMutationType
subscription: RootSubscriptionType
}
```

As far as I know this should be valid.

Start at lib/language-server/providers/schema/file.js, especially FileSchemaProvider.loadFileAndGetDocument, and reproduce the failure with the Absinthe-generated schema shown in the issue. Check how the GraphQL parser handles the leading schema description; done means the extension accepts this schema without the reported syntax error.

Written by the indexing model from the issue text.