Better access into schema contents

[gregsdennis]

Users could benefit from being able to access the data contained in the schema a bit more easily.

Add an extension .Get<T>() where T : IJsonSchemaKeyword to get the indicated keyword.

This can open the door to a set of extensions that directly get the values of the spec-defined keywords, e.g. .GetTitle().

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Additionally, I would like to explore using a dictionary to store the keywords by name instead of in a list. (The Keywords property would be a forwarded property to the Values collection on the keyword, so no breakages.)

This would make accessing keywords faster.