Add support for descriptions to all AST nodes according to the draft specification

[almazik]

Fixes #33
Description parsing added to the following AST Nodes (matches the current draft specification)::

• GraphQLSchemaDefinition
• GraphQLScalarTypeDefinition
• GraphQLObjectTypeDefinition
• GraphQLFieldDefinition
• GraphQLInputValueDefinition
• GraphQLInterfaceTypeDefinition
• GraphQLUnionTypeDefinition
• GraphQLEnumTypeDefinition
• GraphQLEnumValueDefinition
• GraphQLInputObjectTypeDefinition
• GraphQLDirectiveDefinition

This pull request doesn't add the Block Strings descriptions as that should be done on lex parsing level

[sungam3r]

GraphQLDescription node has its own location. Why do you set the start of this location to other nodes?

[sungam3r]

Also I think it would be better to move the GetDescription() inside such ParseXXX methods and remove the parameter. Then the code will be unified with the GetComment() method. How do you think?

[almazik]

GraphQLDescription node has its own location. Why do you set the start of this location to other nodes?

Unlike comments, the description is a part of a formal definition of the parent node. So I followed the approach with other AST nodes. For example, GraphQLObjectTypeDefinition has a name, and so its location is wider than the corresponding GraphQLName's location.

Also I think it would be better to move the GetDescription() inside such ParseXXX methods and remove the parameter. Then the code will be unified with the GetComment() method. How do you think?

I tried to place the description parsing closer to the specific ParseXxx, but there was a problem: when we encounter a string (description), we don't know what kind of node we are parsing yet. So we have to read past the description node in order to find out the name of the entity and branch the logic to the corresponding ParseXxx method. That was the reason why I refactored ParseNamedDefinition into 2 methods: ParseNamedDefinition which parses named nodes without description (and thus immediately knows which node we are at), and ParseNamedDocumentedDefinition which is called when we encounter a string literal (description).

[prowe]

If this PR has no blocking issues, My team would really appreciate this getting merged..

[almazik]

We eventually switched to HotChocolate due to blocking issues in graphql-dotnet and no clear progress in resolving them

[sungam3r]

I am looking forward to reviewing several PRs this month. Too much work, too little free time.

[Shane32]

Superseded by #132