[estimate] Add support for `$reason` modifier

See the discussion here:
https://github.com/AdguardTeam/CoreLibs/issues/1986

It's relatively simple to add it to MV2. With MV3 it is more complicated.

There are several approaches to how it can be implemented, but it seems the easiest way would be to use `webRequest`, catch events when the main frame is blocked and double-check which rule was used to block it using the engine.