Add support for Plugins

Similar to ESLint, we can add plugin support to AGLint. This would allow integrating another library with AGLint through the `plugins` field in the config file. We need to design an interface that plugins must adhere to.

A typical use case for this could be a [dead domains linter](https://github.com/AdguardTeam/DeadDomainsLinter), which is too heavy to be part of the core AGLint but can be made available as an installable plugin.

References:
- https://www.npmjs.com/package/jiti