Added FieldSelectionMap parser

[glen-84]

Summary of the changes (Less than 80 chars)

• Added FieldSelectionMap parser.

---

🚫 Blocked by:

• Parsing of name.{ in SelectedValue graphql/composite-schemas-spec#75

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.34%. Comparing base (7ecc977) to head (acf1d65).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7211      +/-   ##
==========================================
+ Coverage   74.33%   74.34%   +0.01%     
==========================================
  Files        2880     2880              
  Lines      150294   150294              
  Branches    17479    17479              
==========================================
+ Hits       111716   111732      +16     
+ Misses      32762    32744      -18     
- Partials     5816     5818       +2     

Flag	Coverage Δ
unittests	74.34% <ø> (+0.01%)	⬆️

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