Add support for escaped characters in JsonString (for {Try}GetDateTime{Offset}).

[ahsonkhan]

See dotnet/corefx#42574 (comment)

Escaped characters don't work in the current implementation of JsonString (and they should, similar to JsonElement.GetDateTime and Utf8JsonReader.GetDateTime).

string testStr = "\"\\u0031997-07-16T19:20:30.4555555+14:00\"";

using var doc = JsonDocument.Parse(testStr);
Assert.True(doc.RootElement.TryGetDateTime(out DateTime dateTimeVal));

var jsonString = new JsonString(testStr);

// This currently fails, returning false.
Assert.True(jsonString.TryGetDateTime(out dateTimeVal));

See https://github.com/dotnet/corefx/blob/dc6136f34c7d4ebbc493a7bb6cf1d15bc82250e6/src/System.Text.Json/src/System/Text/Json/Reader/Utf8JsonReader.TryGet.cs#L849-L852

https://github.com/dotnet/corefx/blob/dc6136f34c7d4ebbc493a7bb6cf1d15bc82250e6/src/System.Text.Json/src/System/Text/Json/Document/JsonDocument.cs#L665-L669

cc @khellang

[eugenesmlv]

I'm working on this issue.

[eugenesmlv]

@ahsonkhan should I open a pull request in runtime repo?

[ahsonkhan]

@eugenesmlv, yes, please.