FsCodec.SystemTextJson (awaiting UnionConverter)

Adds a full implementation of the FsCodec.Newtonsoft feature-set based on System.Text.Json

NOTE 1: depends on preview edition of System.Text.Json v 5-preview.3
NOTE 2: UnionEncoder has not yet been completed #43

See CHANGELOG

🙏 @ylibrach @NickDarvey for making it happen

Relax class constraint

See CHANGELOG

Final 2.x IEventData/ITimelineEvent interfaces

See CHANGELOG

Add ITimelineEvent.Context

See CHANGELOG

Add StreamName

See CHANGELOG

IUnionEncoder->IEventCodec name change

See CHANGELOG

Minor fix (fixed argument order in FsCodec.Codec.Create)

See CHANGELOG

Add Box.Codec

See CHANGELOG

Polished up/down/mapCausation *.Codec.Create overloads

See CHANGELOG

Initial release

See CHANGELOG

Thanks to @amjjd @eiriktsarpalis for notable contributions to 1.x