LSP switch from `NewtonsoftLanguageServer` to `SystemTextJsonLanguageServer`

[0101]

Newtonsoft is less performant and will be deprecated.

Some work is needed here to support polymorphic (de)serialization which Newtonsoft handles but STJ doesn't.

[charlesroddie]

If performance is a consideration, why not use CBOR?

For the few cases where external APIs require us to use json, we moved to System.Text.Json from Newtonsoft to remove trim warnings, but doing any manual serialisation and deserialization is a huge pain: the library is entirely designed around automatic deserialization using C# types-as-code style and source generators.