Add EqualsIgnoreCase method

https://github.com/linksplatform/Collections/blob/c3381aa586570f5a52dfe12d8954390d82196dae/csharp/Platform.Collections/StringExtensions.cs#L9

Possible names:
```
CasefreeEquals
CaselessEquals
EqualsIgnoreCase
IgnoreCaseEquals
EqualsIgnoringCase
```

At the moment, `EqualsIgnoreCase` name is the most popular on GitHub.