Added FieldSelectionMap parser

[glen-84]

Summary of the changes (Less than 80 chars)

• Added FieldSelectionMap parser.

---

Notes:

1. I'm not sure if the FieldSelectionMapSyntaxSerializer is correct, taking over the visitation and then returning Skip.
2. Since the parser is similar to the main GraphQL parser, it is affected by the same issue with locations (incorrect End values).
3. The following types could potentially be shared.
   • Location (if the ISyntaxNode crefs were removed)
   • TokenInfo (if the ISyntaxNode crefs were removed)
   • ISyntaxVisitorAction
   • SyntaxVisitorActionKind
   • SyntaxVisitorActionExtensions
   • *SyntaxVisitorAction