updating for Try .NET -- System.String.*

[WilliamAntonRohm]

per https://dev.azure.com/mseng/TechnicalContent/_workitems/edit/1558837/

Related dotnet-api-docs PR is dotnet/dotnet-api-docs#2971

[mairaw]

@WilliamAntonRohm I've removed the changes here because they depend on culture, so we shouldn't touch those. Make sure that the changes in dotnet-api-docs are also reverted.

[mairaw]

Thanks @WilliamAntonRohm. The changes overall looked great! I've made a few fixes unrelated to your changes that were related to our C# coding guidelines (https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/coding-style.md), which I've noticed were not being followed here.
Also, I had to undo the changes to a few samples that depended on the current culture, which made them have different results when run under the try .net engine (you can test them at https://try.dot.net/).
So whatever changes you made to make those samples interactive will have to be removed and include notes reviewed to see if we still have right ones.