Json prototype - transformation API

[kasiabulat]

I added following transforming API (and accompanying scenarios):

• DeepCopy method in JsonElement allowing to change JsonElement and JsonDocument into JsonNode recursively transforming all of the elements
• AsJsonElement method in JsonNode allowing to change JsonNode into JsonElement with IsImmutable property set to false
• IsImmutable property informing if JsonElement is keeping JsonDocument or JsonNode underneath
• Parse(string) in JsonNode to be able to parse Json string right into JsonNode if user knows he/she wants mutable version
• DeepCopy in JsonNode to make a copy of the whole tree
• GetNode and TryGetNode in JsonNode allowing to retrieve it from JsonElement

Issues addressed from previous PR:

• I added Sort method to JsonArray and accompanying scenarios
• From ArrayList methods I considered only LastIndexOf useful and consistent with IndexOf, but I still need to write scenarios for both

Still open question from previous PR:

• Do we want to add recursive equals to JsonArray and JsonObject?

cc: @joperezr @bartonjs @ericstj @ahsonkhan @terrajobst

[kasiabulat]

I will add more tests in subsequent PR.

[kasiabulat]

I created the issue for my project: https://github.com/dotnet/corefx/issues/39922