[validator] Implement "Fragment Spread Type Existence".

[codecov[bot]]

Codecov Report

Merging #185 into master will increase coverage by 0.2%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #185     +/-   ##
=========================================
+ Coverage   81.43%   81.63%   +0.2%     
=========================================
  Files         105      106      +1     
  Lines        9135     9193     +58     
=========================================
+ Hits         7439     7505     +66     
+ Misses       1385     1381      -4     
+ Partials      311      307      -4

Impacted Files	Coverage Δ
graphql/validator/rules/unique_fragment_names.go	100% <ø> (ø)	:arrow_up:
graphql/validator/walk.go	97.54% <100%> (+4.81%)	:arrow_up:
...raphql/validator/rules/lone_anonymous_operation.go	100% <100%> (ø)	:arrow_up:
graphql/schema.go	86.27% <100%> (+0.36%)	:arrow_up:
...phql/validator/rules/single_field_subscriptions.go	100% <100%> (ø)	:arrow_up:
graphql/validator/rules/known_type_names.go	100% <100%> (ø)
graphql/validator/rules/init.go	100% <100%> (ø)	:arrow_up:
graphql/validator/rules/unique_operation_names.go	100% <100%> (ø)	:arrow_up:
graphql/internal/validator/messages.go	100% <100%> (ø)	:arrow_up:
graphql/validator/validation_context.go	95.45% <100%> (+2.35%)	:arrow_up:
... and 2 more

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