Add isClass function

[aleclarson]

The isClass type guard would check for a function that must be called with the new operator. It would only check for native JavaScript classes, not old-school function MyClass() {} definitions. This is a fine concession, since such old-school classes are very uncommon, and they typically check for the existence of this and do a recursive new call if this is an unexpected value.

In my own type guard library, I implemented isClass like this:

export type Class<T = unknown> = new (...args: any[]) => T

export const isClass = (value: unknown): value is Class => isFunction(value) && value.toString().startsWith('class ')

The only time this breaks is if you're compiling down to an ES target that doesn't support native class syntax (ES5 and below). We should recommend not using isClass if you plan on targeting such an environment, but Radashi only supports ES2017+, so adding such an isClass implementation is acceptable.

[aleclarson]

In progress at #239