Write a GraphQL Document Parser to Use in Ballerina GraphQL Engine

[ThisaruGuruge]

Description:
Ballerina GraphQL engine should be able to read and retrieve various information from a given GraphQL document, including the operation type, fields for each operation, inputs, operation names, etc. Therefore, a parser should be written in order to the engine to retrieve this information.

In the first step Parser should be able to handle single-queried, shorthand-notationed documents.

Subtask of #825

[ThisaruGuruge]

Completed