Newtonsoft.Json -> System.Text.Json

[beppemarazzi]

First quick and dirty attempt to migrate from Newtonsoft.Json to System.Text.Json
See #22 and datalust/seq-api#125

@nblumhardt WARNING: this PR is for your reference only: please review it carefully!!!!
All the tests on this repo are passing, but probably there are some API names are no more correct.
i.e.

serilog-formatting-compact-reader/src/Serilog.Formatting.Compact.Reader/LogEventReader.cs

Line 109 in b678843

public static LogEvent ReadFromJObject(JObject jObject)

now becomes

serilog-formatting-compact-reader/src/Serilog.Formatting.Compact.Reader/LogEventReader.cs

Line 123 in 4f1918c

public static LogEvent ReadFromJObject(in JsonElement jObject)

but probably the name ReadFromJsonElement is more appropriated than ReadFromJObject

obviously there are also breaking changes when the public API references Newtonsoft.Json

[nblumhardt]

This looks like a great start, thank you Giuseppe! I'll dig thoroughly into this and try to push it forward before moving over to the other PR.

We'll shortly be releasing a new version of Serilog.Formatting.Compact.Reader to support Serilog 3.1 (with the addition of built-in trace and span ids), so the best time to merge will be after the dust has settled on that one, in a month or so, I think.

[Numpsy]

Is this likely to go any further in the near future? (not sure how risky it is, just wondering in the context of a major version bump for Serilog 4)

[nblumhardt]

I don't think this will be achievable in the post-Serilog-4 package rollout, needs more analysis.