Implement simple and lightweight api functions for JS rules

[confessor-adguard]

Original comment #1099 (comment).

Must-have functions:

1. window load handler (like AG_onLoad)

$(function() { /* here goes the code */ });

Optional functions:

1. simple function to remove elements by selector:

$('someelement').remove();

2. closest parent selector:

$('element').closest('div')

3. children selector:

$('element').find('selector')

Reference: https://github.com/finom/balalaika

[ameshkov]

In light of possible migration to scriptlets, we do not need it anymore:
AdguardTeam/AdguardFilters#4335