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
• SelectedListValue is not referenced graphql/composite-schemas-spec#76

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 74.37%. Comparing base (9021541) to head (fcd3049).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7211      +/-   ##
==========================================
+ Coverage   74.35%   74.37%   +0.02%     
==========================================
  Files        2871     2872       +1     
  Lines      149806   149876      +70     
  Branches    17417    17422       +5     
==========================================
+ Hits       111385   111475      +90     
+ Misses      32619    32599      -20     
  Partials     5802     5802              

Flag	Coverage Δ
unittests	74.37% <ø> (+0.02%)	⬆️

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