Fix 'isValidCallback' — returns false for functions without a prototype

[AdamWr]

Related issue - AdguardTeam/AdguardFilters#231983, though there is another problem that scriptlet is not injected into iframe.

Steps to reproduce:

1. Add to user rules:

example.org#%#//scriptlet('adjust-setTimeout', '/./', '7000', '0.001')

2. Navigate to - https://example.org/
3. Run in console:

const logTest = () => console.log('Test');
Object.setPrototypeOf(logTest, {foo: 1});
setTimeout(logTest, 7000)

Delay in setTimeout is not adjusted because callback instanceof Function returns false in:

Scriptlets/src/helpers/prevent-utils.ts

Lines 15 to 16 in 3ba7a33

	export const isValidCallback = (callback: unknown): boolean => {
	return callback instanceof Function

I guess we can change it to typeof callback === 'function'.

---

And probably for such cases we need to use Function.prototype.toString.call(callback); instead of callback.toString().
The former returns () => console.log('Test'), but the latter [object Function] in this case.