Add support for Fluent Assertions v8

[adrianiftode]

Add support for FluentAssertions v 8.0.0

Will maintain both Fluent Assertions v7 FOSS and Paid Fluent Assertions v8

• code for v8 will always be in master-fluent-assertions-v8
• new nuget packages: FluentAssertions.Web.v8 and FluentAssertions.Web.v8.Serializers.NewtonsoftJson
• still thinking on the name (FluentAssertions.Web.xceed maybe?)
• might not be able to use the nuget FluentAssertions prefix, to ask for permission, it that's even possible

Addressing #130 and fluentassertions/fluentassertions#2943

[adrianiftode]

I'll probably change the approach to still use master, but add conditional preprocessor directives to handle v7 vs v8