Fix to use snake_case rule when removing method prefixes

[hadashiA]

refs: #57

GroupingExtensionMethods removeUntilTypeName does not seem to work as per the README.

This is because it removes the TypeName of CamelCase, but ignores the type_name of snake_case.

I've fixed it to use snake_case.

Before:

public static void CounterContextInsert(this ref global::CsBindgen.CounterContext @context, int @value)
{
    NativeMethods.counter_context_insert((global::CsBindgen.CounterContext*)Unsafe.AsPointer(ref @context), @value);
}

After:

public static void Insert(this ref global::CsBindgen.CounterContext @context, int @value)
{
    NativeMethods.counter_context_insert((global::CsBindgen.CounterContext*)Unsafe.AsPointer(ref @context), @value);
}