Clean-up code by introducing String compare helpers

[Corniel]

To improve on the readability when comparing strings case insensitive, I've created 3 extension methods:

namespace System;

internal static class BuildalyzerStringExtensions
{
    public static bool IsMatch(this string? self, string? value);
    public static bool IsMatchStart(this string self, string value);
    public static bool IsMatchEnd(this string self, string value);
}

For obvious reasons this helpers are kept internal, as this should not be a deliverable of Buildalyzer.

[Corniel]

@phmonte Can we agree on the usefulness of this?