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 74.39%. Comparing base (61f1f59) to head (621d3a3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7211   +/-   ##
=======================================
  Coverage   74.39%   74.39%           
=======================================
  Files        2876     2876           
  Lines      150071   150071           
  Branches    17450    17450           
=======================================
+ Hits       111641   111644    +3     
+ Misses      32613    32611    -2     
+ Partials     5817     5816    -1     

Flag	Coverage Δ
unittests	74.39% <ø> (+<0.01%)	⬆️

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