Replace Json.Net with System.Text.Json

[taooceros]

close #252

[jjw24]

hey @taooceros have you tested this new serializer working with settings and our default plugins?

[taooceros]

Yes, I have tested with default plugin and the setting, and found one issue. Originally, Json.Net will use ignore null value to load setting.json, while System.Text.Json doesn't successfully load that with the setting, but successfully load it without ignore null value.
Also, it doesn't support manually property ordering, so I put the plugins setting to the later part of the Settings.cs

[taooceros]

It seems work fine.

[jjw24]

I am assuming these options and comment are no longer applicable?

[taooceros]

yes, the default behavior of System.Text.Json is replace, and ignore doesn't work well as my comment before.