Added IsMutation to ICompiledQuery.

[grokys]

Allows one to check programatically whether a query is a mutation query.

Needed for GraphQL caching in github/VisualStudio#2181

[codecov]

Codecov Report

Merging #186 into master will decrease coverage by 29.08%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master     #186       +/-   ##
===========================================
- Coverage   88.53%   59.45%   -29.09%     
===========================================
  Files          73       55       -18     
  Lines        3575     2513     -1062     
===========================================
- Hits         3165     1494     -1671     
- Misses        410     1019      +609

Impacted Files	Coverage Δ
Octokit.GraphQL.Core/Core/PagedQuery.cs	0% <0%> (-97.71%)	⬇️
Octokit.GraphQL.Core/Core/SimpleQuery.cs	30.23% <0%> (-26.92%)	⬇️
...phQL.Core/Core/Introspection/IntrospectionQuery.cs	0% <0%> (-100%)	⬇️
Octokit.GraphQL.Core/Connection.cs	0% <0%> (-100%)	⬇️
...kit.GraphQL.Core/Core/Syntax/VariableDefinition.cs	0% <0%> (-100%)	⬇️
...phQL.Core/Core/Builders/SubqueryPagerExpression.cs	0% <0%> (-100%)	⬇️
Octokit.GraphQL.Core/Variable.cs	0% <0%> (-100%)	⬇️
...it.GraphQL.Core/Core/GraphQLIdentifierAttribute.cs	0% <0%> (-100%)	⬇️
Octokit.GraphQL.Core/PagingConnectionExtensions.cs	0% <0%> (-100%)	⬇️
...tokit.GraphQL.Core/QueryableInterfaceExtensions.cs	0% <0%> (-100%)	⬇️
... and 40 more