Add multiple items from `citynames`

[dninemfive]

Items to add from the citynames repo:

• CollectionExtensions.cs
  • .WeightedRandomIndex()?
  • .CrossJoin()?
  • .To()
• FileUtilityExtensions.cs
  • .FileNameSafe()
• JsonExtensions.cs - make generic to use with e.g. YamlDotNet?
  • .FirstArrayElement()
  • .NullablyGetProperty()
• LogUtils.cs - convert to work with Microsoft.Extensions.Logging
  • .Method()
  • .ReadableString()
  • .ReadableTypeString()
  • .LogInvocation()
  • .LogAndTime()
• StatisticsUtils.cs
  • .Erf()?
  • .Lerp()
• StringExtensions.cs