Add support for OfType, Is, As and Cast

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #249 into master will decrease coverage by 0.07%.
The diff coverage is 92.01%.

@@            Coverage Diff            @@
##           master    #249      +/-   ##
=========================================
- Coverage   96.57%   96.5%   -0.08%     
=========================================
  Files          41      41              
  Lines        6925    7206     +281     
=========================================
+ Hits         6688    6954     +266     
- Misses        237     252      +15

Impacted Files	Coverage Δ
....Core/Compatibility/CustomTypeBuilderExtensions.cs	50% <ø> (-50%)	⬇️
...Providers/AbstractDynamicLinqCustomTypeProvider.cs	71.23% <0%> (-28.77%)	⬇️
...eProviders/DefaultDynamicLinqCustomTypeProvider.cs	85.71% <0%> (+85.71%)	⬆️
.../System.Linq.Dynamic.Core/DefaultAssemblyHelper.cs	100% <100%> (+100%)	⬆️
.../System.Linq.Dynamic.Core/Parser/KeywordsHelper.cs	100% <100%> (ø)	⬆️
src/System.Linq.Dynamic.Core/ParsingConfig.cs	100% <100%> (ø)	⬆️
...em.Linq.Dynamic.Core/DynamicQueryableExtensions.cs	100% <100%> (ø)	⬆️
...namic.Core/Parser/SupportedMethods/MethodFinder.cs	100% <100%> (ø)	⬆️
src/System.Linq.Dynamic.Core/Parser/TypeFinder.cs	100% <100%> (ø)
...ystem.Linq.Dynamic.Core/DynamicExpressionParser.cs	87.83% <100%> (ø)	⬆️
... and 8 more

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