Add String.Contains() and StringComparison enum

[Gaulomatic]

Hi,

I have added a static String.Contains() method with an optional parameter of a also added StringComparison enum. Makes string comparisons much more C#-like.

[sevensc]

Good idea, but i would rather create a prototype for strings.
usage something like this.

'this'.Contains('hi');

implementation something like this.

String.prototype.Contains = function (value: string): boolean {
    return this.indexOf(value) !== -1;
}

[Gaulomatic]

I thought about this as well and although (from an object orientated perspective) this makes absolute sense, I decided against it. The RxJS team also moved away from prototype manipulation. I watched a talk about this topic in the aftermath of the 6.0 release. I can't quite remember why they went this route, but from what I can tell, it can be a bit complicated to make sure to get the 'new' prototype and get IntelliSense for TypeScript. I extended the error object for an app and declared an interface. But you need to make sure, that the prototype is indeed modified, because the TS compiler won't complain (because he just don't really know).

EDIT:
To make clear what I am talking about: The TS compiler in WebStorm and VSCode is by now already confused when using your String class, it messes up the imports all the time, because there is a String whatever somewhere under the hood.

C# has for the most part static and instance implementations as well, so you do not need to check for null beforehand. This makes the using code look cleaner. So maybe we could use both as well? It's just a preference thing at the end of the day.

[sevensc]

Could you do a test for the Contains Method?