Update Newtonsoft.Json package to version 12.0.2

[grochocki]

After publishing final feature update supporting VS2017 (#480), we will be unblocked form updating Newtonsoft.Json to version 12.0.2. More information here: Using Newtonsoft.Json in a Visual Studio extension.

[hansmbakker]

Question out of interest: why is XamlStyler sticking with NewtonSoft.JSON now that System.Text.JSON is available?

[mrlacey]

Question out of interest: why is XamlStyler sticking with NewtonSoft.JSON now that System.Text.JSON is available?

Visual Studio uses Newtonsoft.Json internally and ships with a version. Having extensions ship with a different version of Json.NET can cause hard to debug issues.
Obviously, XamlStyler works with more than just VS, but using different serialization libraries in different versions of the tool would introduce new complexities.

Why not remove Json.NET entirely? I assume it comes down to not wanting to create work with no immediate benefit (other than supporting installation on older VS versions).

[hansmbakker]

Why not remove Json.NET entirely

@mrlacey this was what I meant in #481 (comment)